Is it permissible to eat chocolate that a teacher bought as a punishment for students who spoke Arabic or were late after the break?

The financial penalties imposed by the teacher on students for disciplinary reasons or for late attendance are based on the issue of the permissibility of ta'zeer (disciplinary punishment) by seizing money. Ta'zeer by seizing money is a matter of dispute among scholars, with most of them deeming it impermissible. This is the view of the four madhhabs, and Al-Sawi reported a consensus on this. Those who permit ta'zeer by seizing money restrict it specifically to the wali al-amr (the legitimate ruler or authority). Therefore, what the teacher imposed on the students is not permissible.

As for the food you have, if it is possible to return it to its owners, you must do so. Otherwise, donate it as charity on their behalf.