The above-mentioned manner of correcting a student's grammar is very diplomatic and effective with older students, and certainly the wisest approach. For those who are much younger, however, offense is not so easily taken when a teacher says "gone" right away after the student misuses the verb "went." If the tone of the teacher is no different from the usual voice he/she uses, most young students will repeat the correct way and move on. As a ninth grade teacher, you may want to explain that the state test will demand correct usage of subjects and predicates, pronouns and antecedents; so, you are trying to prepare them. Many ninth graders are not offended by correction when they understand the teacher's motives are not to embarrass them. Sometimes giving bonus points to those who detect grammatical errors in the dialogue of certain short stories helps to develop a critical eye on the part of the student.+
Now, on written work such as essays, etc. the time-honored use of two grades [content and mechanics] directs the student to understand that standard usage is important and demanded. Not counting off for everything, but writing the appropriate way above the occasional erroneous phrase or sentence is a kinder, gentler method. Sometimes it works--if the student really wants to learn. Marking so much can discourage ninth graders. Is it not odd to watch the old movies in which the instructor publicly berates the student? Times have changed!
Certainly, when an entire faculty enforces standard usage in the classroom and on written work, progress can be made. On the other hand, when some teachers themselves make use of substandard forms, it is indeed difficult to impress upon students the need for standard English in their daily lives.
I assume that you are talking about grammatical mistakes that they make while speaking in class (as opposed to written mistakes). My approach to that is to try to correct them without actually saying they said something wrong and without (I hope) calling attention to the person who made the mistake.
For example, if we are having a class discussion and a student says "the President should have went..." I will not say anything to the student. Instead, I will restate his or her comment using proper grammar. I will say "okay, so Student X says that the President should have gone...".
By doing this, I hope to show the students the correct usage without embarrassing the student who made the mistake