OK, let’s play a little. What “severe negative influence” is there to the rest of the class for three weeks, for talking to a neighbor outside school?
I don’t agree with J/P.M. That there’s a direct negative influence, unless the boy talks to his classmates about it (which is possible, since he was in a public place and had no “busha”).
However, if such a public violation of a yeshiva’s standards is allowed to go unpunished, the standard becomes unenforceable.
And, let’s face it, it’s a black eye for the yeshiva (which is why the menahel should not make such a decision unilaterally, since he has “negiyus”).