I have taught at the high school and college level, and there is no difference in my planning or classroom strategies. How I handle physical and mental differences is sometimes different, though.
The overarching mindset that I have is that all students have something wonderful to contribute to my classroom, no matter what their disabilities or differences are, and it is up to me to find that something. With each new class, it can take some time to find out what contributions each student can make, but as I begin to know my students, I try to offer choices in activities and start with some group projects, so everyone can be engaged in some way, and also so that my students can get to know one another's strengths and weaknesses. The idea is to make my classroom a community in which we all take care of one another. For example, in an English class, after the students have been assigned a brief fiction text, I will offer the choice of writing a summarizing tweet (which is more difficult than it sounds), illustrating the text, writing a letter of recommendation for a character, or acting out a part of the story. Students can choose which group they will be in for this purpose, so that they are more driven by assignment preference than by preference to be with particular other students. This helps to break up cliques and to minimize any tendency to reject those who have differences of any sort. Then I can walk around, and I watch and listen carefully, with a little bit of guidance here and there. Once I know what the strengths and weaknesses are, I can play to the strengths in my planning and plan for the weaknesses, too, by including activities that students can shine at and by careful grouping so that students can help others in need. I had a severely ADHD student who was a bit of an expert in military history, for instance, so when I found that out, I made sure that we needed him in some of our assignments.
It is virtually impossible to not be open about physical disabilities. For the most part, they are observable to me and to the other students. Even for those that are not immediately obvious, for example, diabetes, it is difficult to accommodate without being open, since a diabetic student might have to leave the classroom to check blood sugar levels and to take insulin. Since everyone knows about a disability, it is best to be open and frank about it, and if possible, to use the disability as an opportunity for educating the class. When my daughter was in school, there was a student with severe and uncontrolled epilepsy, so the school decided to educate everyone about epilepsy, which was much the best approach, in my opinion. The student was pleased to talk openly about her problem, and I think most students are. A few terms ago, I had a student in a wheelchair. And before the first class was over, students were knocking themselves out trying to help him. I spoke with him privately about whether or not he welcomed all the help, since some students do not, but as it turned out, he was quite pleased to get it, and felt better about the frankness, as opposed to people seeing him and turning their eyes away. Activities for the physically impaired should be dealt with head-on. A blind student will need to be read to. A hearing impaired student can teach us to sign. There is no point in not being open about any of this. And a teacher is not responsible for a breach of privacy or confidentiality for a condition everyone knows about.
Mental differences, which include disabilities and giftedness, are trickier, since our knowledge of these is often confidential, and we must be more subtle in our strategies. Again, though, the idea is to provide activities that draw on strengths and help with weaknesses. If I offer a wide enough "menu" of activities for a class, students have no idea that my plan is to engage a student with a disability in a particular way or to offer additional enrichment for the gifted student. Careful grouping allows students to support one another, too. If their grades rest or fall on the product of the group, they are highly motivated to help one another, and it is my experience that they do. When calling upon students in a classroom discussion, I make sure I know who can answer well, so I can help a student shine. A wrong answer is something I have to be clear about, all the while turning it into a helpful contribution in some way. A student who offers the wrong character name, for instance, can be told that is not the character, but that I'm glad he brought it up because we haven't talked enough about that character. No matter what, it is possible to make a student feel it was worth raising a hand and speaking up. Having said that, I should also say that many students with mental disabilities are quite open about them today, sharing a condition with fellow students, joking about having missed the meds for the day or raising a hand to say they just can't sit still any longer. This attitude, of course, is the easiest to deal with, since the student has already called attention to his or her disability. And I think this is beginning to be a trend.
In summary, the best approach to take is to believe that every student has something to offer. All you have to do is figure out what that is and incorporate activities and assignments that allow this to happen. For physical impairments, there is seldom a way to avoid being overt in accommodation, but for mental disabilities or differences, we must take care and be subtle about this, again, through the use of activities and assignments, but with a decreasing need today to worry about privacy, since so many students have already waived that privacy on their own.
The ability to provide accommodations without drawing attention to the student receiving the accommodation is dependent on the accommodation being provided, but there are things you can do as the teacher to make sure the student receives as little "special" attention as possible. One way is to incorporate varying levels of support into your lessons. This requires more planning, but reduces the chance of singling out the student who need the accommodation, while increasing the chance of helping other struggling students. One method of achieving this is the use of choice based assignments and/or contracts. Everyone receives the same paper containing directions for several different options for completing the assignment and students can choose the option (with teacher guidance, if necessary) that best fits the individual's strengths and weaknesses. The use of small groups or centers can also achieve this goal. The lessons/activities can be planned around the needs of a specific group of students, whether it is extra practice on a skill or an extension of what has already been mastered.
Another way to provide accommodations is to have supplies such as highlighters, colored pencils, rulers, reading trackers/guides, blank scrap paper, graphic organizers, dictionaries, vocabulary lists, etc readily available to anyone who chooses to use them or by providing a "toolkit" containing such items to specific students to keep in their desk or locker.
Finally, some accommodations, such as having items read aloud or the use of a scribe, are harder to hide. These types of accommodations often require the use of technology or extra support from the teacher or an assistant. It is important to talk to the individual student, in an age appropriate manner, to discuss why the accommodation is being provided and the get the student's opinion as to the best way to provide the accommodation. It is also important to talk to the class as a whole, as the need arises, to reinforce the idea that everyone has strengths and weaknesses and part of growing up is understanding and accepting this. "Fair" doesn't necessarily mean everyone gets the same thing, it means everyone receives what they need to be successful.
The answer to this question is really contingent upon the need of the student. If a student requires accommodations like leaving the room for testing, small group assessment or read aloud, it is difficult for that student to receive the accommodation without it being noticed. Having said that, have a conversation with the student prior to assessment day and have them go to their alternative location directly instead of leaving the room at the start.
Other accommodations may be easier to have go unnoticed. If you have a student who gets an alternative assignment or modified test, being prepared is key. Have these modifications ready before test day and hand them out while handing out everyone else's. If you have students who get modifications like being able to use notes in assessments or being able to get a copy of the teacher's notes, strategic seating can help reduce the chances of other students noticing.
Once the introduction segment of the lesson has been given, depending on the accommodations they will follow. The student in my class usually needs the directions listed and repeated to him one on one. I walk over to the student's desk and I give him the piece of paper with the directions and then I proceed to read the list aloud. The student's are occupied completing the task on hand and I feel this is most effective in my classroom.