OP, I agree with many of the posters here that we don’t really know the impact or lack of impact had on your classmates, since that is something not always shown on the surface. I personally have certain views on the topic of communication between boys and girls that aren’t quite in line with some people’s views here, but that doesn’t change the fact that a school rule was broken. But, I do think that in general, a Principal or Rabbi in a school should really think carefully before making a decision that can possible change a student’s life, because they may be held responsible in heaven for their decision.
Concerning something else brought up here, I don’t think going to an MO school will solve anything. If anything, it’ll make things worse, both for this student, and fellow classmates. I PERSONALLY know of so many kids from more religious schools who were expelled, went to more modern schools, and ended in far worse trouble, and caused other classmates to stumble, eventually giving the class, and even the entire school, a bad name.
I think MO schools should NOT accept students who were expelled from more religious schools. There are schools specially made for students like that in most large cities.
But of course, my opinion on this is simply based on what I saw. So if people disagree with me, that is perfectly fine, because they may have seen things work out differently, and I understand that.
May you find your path back to G-d quickly and easily.