It is forbidden to speak loshon hara about children. While most people understand that “kids are kids” and the Torah does not hold children responsible for their action, if one speaks about a child in a manner that either the speaker or listener sees as derogatory, then the laws of shmiras haloshon have been violated.
It is also forbidden to say or write something about a child that potentially could prove harmful to him.
A teacher who is prepared to enter a negative comment on a student’s record should pause to contemplate the effect that entry might have on the student’s future educational and employment opportunities.
Likewise, teachers should exercise caution and fairness when conveying their feelings concerning a student to his teacher for the coming year.
Sefer Chofetz Chaim, 26 Tishrei, page 100