The boy is a teenager, he was most probably aware of the rules. HOWEVER even the owner of an animal is warned three times before being liable for being a muad. Perhaps the boy does not think talking to girls is such a big deal, a teacher or menahale must try reaching out educating and talking before using expulsion.
A far better idea would have been to
A) find out why the boy was hanging out
B) after finding out such go over the halachos of tzniyus inside to make the boy realize that it is truly wrong. Not just some rules the school likes to enforce
C) Then warn the boy that such further behavior cannot be tolerated as it is unbecoming a true yeshiva bochur.
D)The informer should have spoken to the boy or if not viable to the boys parents.By speaking to the principle first, the boy was already shechted by the time he came to the office.As who knows what the informer said.
The life of a teenager is very hard and complex.A yeshiva boy works harder then most adults from seven in the morning till late at night and no sundays.Additionally, they must constantly follow certain rules which they do not understand. We as a society demand a lot from them in turn we must be extremely understanding of them
We cannot just press the Auto-Pilot button and assume that teenagers should just listen to everything just because it is right or came from an adult or Rebbe. How many Mussar sefarim are there? Dozens! Because it is so easy for adults to go astray. Why expect more from a teenager? Remember a teenager, does not fully comprehend things yet, is a work in progress and is in a constant state of pressure to succeed.Teenagers today have a very pressured life Yet, the chinuch model has generally not changed to help them, and we expect so much from them.Many times unfortunately, for problems big and small we just sweep them under the rug or completely asser certain activities without an effective way of dealing with them.What harm would have been done by this Rosh Yeshiva in a loving way learning together with this boy the Halachos of tzniyus with a warning that this behavior should not happen again?
May the writer find Hatzlacha in all future endeavors