As a teacher, I understand your concern. That said, one can look at the placement of the most challenging students within his or her classroom in two ways.
First, one may think that the person/people in charge of student assignment may "have it out for you." Typically, this is the negative way to look at things.
Second, one could look at the placement of challenging students within his or her classroom with a positive outlook. Those who have successfully placed challenging students within the walls of a specific teacher's classroom will continue to do what is best for those students (by placing them with the best teacher). I tend to believe more in this choice.
As for the steps which can be taken to insure the number of challenging students is reduced, it really depends. First, if the school does random class assignment, there may be nothing one can do. Students are assigned by computer to the classes and times which best suit their schedule needs. Second, one could speak with those responsible for assigning students to certain classes. One could voice his or her opinion regarding the number of challenging students in a single class. Third, one could go administration to voice his or her concern regarding the number of challenging students in a single class. Lastly, one could ask for a co-teacher (usually one who works with Special Education). If none of these work in alleviating the number of challenging students see above (some teachers and students are simple meant to be together because of success levels within the walls of the teacher's classroom).
With regard to student placement, one must first ask the question of what grades and/or classes you are referring to. In elementary grades where subjects are taught in an integrated manner, multiple factors make up the decisions on where to place students. Examples of this would be race, gender, previous educational experiences, behavior records, choices of parents, etc. On the other hand, in junior high and high school settings, student placement is based mainly on schedule constraints and educational requirements.
In any setting, teachers have little control over who is placed in their classes. First, my suggestion would be to take a positive approach to whoever is placed in your class. Do not take the word of any previous teacher or administrator, but instead, give students and yourself time to adjust to new parameters and make your own judgements based on your experiences during the new year. Second, you need to keep accurate and up-to-date anecdotal records on behavior, parent contacts, office referrals, grades, situations with peers, etc. so that you can justify any situation that may arise. Third, if you feel that your classroom has a mix of students that cannot learn together successfully, talk with an administrator on what could be done to help the students. Never make the situation about you as a teacher--always direct any discussion toward the successes and/or failures of the students.