In a certain yeshiva the menahel was threatening bochurim to tell on other bochurim about who damaged something in the yeshiva.
Is it against the laws of Loshon hora to tell?
What about in other similar situations?
The Igros Moshe YD 2:103 writes that it is a deplorable act for a Rebbi to ask his student to tell on another student who did something bad,and it will lessen the seriousness of loshen harah. The Chafetz Chaim in Hilchos Loshen Harah 5 writes that even if a father or rebbi asks him to say lashon harah it is prohibited. However, in a case of to'eles it is permitted just like in any other to'eles case. (See Lashon Harah 10:1 [English].) It would seem this is a case to case basis.
There's an argument between R' Moshe Feinstein and R' Moshe Shternbuch.
R' Moshe Feinstein (as mentioned by sam) says that it's disgusting , as (practically) the teacher is teaching that Lashon Hara is permitted. (or at least not so bad). Even though the Gemara reported instances when Amorayim would inform on their friends to their teacher, there the students did it out of their own volition.
R' Moshe Shternbuch (Teshuvos VeHanhagos 1:539) says that one is allowed to, just one has to explain the stringency of Lashon Hara and the laws of Toeles first.
Moreover, Rabbi Chaim Cohen says that even according to Rav Moshe Feinstein the problem would be only when one makes the child accustomed to tatteling. On the other hand, once in a while one can ask a student can come and tattle on another.
However, he finishes off that one should still be Machmir like the Igros Moshe, and says that this may be the reason people don't care for Lashon Hara - They were educated in it since their Cheder days!!