One thing I think a menahel and Rosh “could” do in such a situation (since some said there was no choice because he would influence others negatively) is to call a parents meeting of that grade. Both the Rosh and the student should present their case and the boy should then leave. The Rosh should put the achrius on the other parents. He should leave the room and tell them if they have halachic sheilos they should come find him in his office.
I am sure if the whole parent body of the graduating class heard the situation they would come up with a solution that would NOT destroy the boy’s life. Firstly, many of them probably know the boy well, much better than the “moser” involved. Secondly, the boys are already heading out the door and off to E”Y, in most cases. They are going to have to learn to trust their own boys in various situations and know that if they have an ehrlich serious son that is who he will remain. They will also realize that they don’t have a clue who their boys will meet in E”Y, and what they will do. So if they judge this boy so harshly and kick him out, is this the verdict and consequence that they are hanging on their own son’s necks if they slip up even a little on their own?
Also note that if this “moser” called the boy’s yeshiva she probably also called the girl’s school. That means that she ruined two kid’s lives over something that was firstly none of her business and secondly probably no big deal. It would be a totally different ball game, if out of concern she spoke to his and to her parents and mentioned that “it might not be appropriate” and they might want to be aware. So yes she is a “moser” of the worst kind.
Getting back to the parents, if someone wanted to check out the truth of the situation, one of the mothers could have quietly and gently met with the girl and heard her version as well. And out of concern for her also explain how such an seemingly innocent act can snowball into ruining two lives. So I suggest that such a decision should have been made by the parent body of the graduating class and the Rosh should abide by that decision.
In addition, if any child in any school knew that if they broke such a rule that the community thought was so horrific and they would have to go in front of the entire grade’s parents to explain themselves in order for a decision to be made whether or not they can remain in school, that in itself would be a more effective means of consequence. No frum child understands how another child can just go OTD. No frum child understands how one can just lose their bitachon and emunah like the drop of a hat. No one believes it will happen to them. So they may or may not take getting kicked out of school as a joke or a serious consequence because after all they can always fall back on the GED. But no child would want to have to go in front of their own parents and their friends’ parents explaining their mistake and beg for another chance. That is scary.