Shouldn't Ms. G NOT go with them to their junior + senior year?

If she made such a difference in their lives, then after the students have been with her for two years, shouldn't she move on to teach new students so she can stop THEM from hating & killing eachother?
and also she said she taught 9 & 10 grade english. so wouldn't she of had more classes than just the class that was focused on in the movie unless the school has a weird system..?
when she was trying to get permission to teach them junior/senior year, they kept saying that without her class "they'd go back to their old ways" but they can always go see ms. G, they'll still be at the same school. and if they meant old ways as in that their new teachers would treat them like they were treated before Ms. G (although any teacher that wasn't ms. G still treated them the old way since ms. G only taught them english, the students have other classes), than i guess that makes some sense. because since they don't like the teacher and the teacher treats them like they're stupid than they would care less about education.
but if 'old ways' they meant they would go back to hating/killing eachother because of race, well would that really happen? from fights breaking out in the classroom the first day to being basically best friends - calling eachother their "family".. thats a HUGE change, and they wouldn't just "go back" without their teacher.
wouldn't it be better for Ms. G to change the lives of MORE students than staying with that class for all four years, and then leaving to go teach at a college? i mean she must've had classes other than them, but how come they're never talked about? wouldn't they have been treated the same way by ms. g?
now don't get me wrong i actually found it to be a very good movie, but i don't get why they fought so hard/found it so important for her to continue teaching them junior&senior year and what about all her other classes?