Lashon Harah Teacher asks student to tattle on others?

Igros Moshe (YD II #103) Question Can a teacher to tell a student to inform on misbehavior of other students?Answer: This is a repulsive and disgusting thing because it minimizes the seriousness of lashon harah. If this is merely to chastise them for the sake of Heaven because this might improve their behavior then it is possibly permitted.  However that is only when the student volunteers to tell the teacher the information but not when the teacher commands him to tell. We are not on the level that this information can possibly be considered as given for pure motivation (for the sake of Heaven) even for adults and surely not with children.

 Igros Moshe (YD IV #30.1) Question The teacher's request that students inform on other students is obviously because the teacher thinks the information is needed to benefit the students so why is that disgusting and causes lashon harah to be minimized?  This is what the Chofetz Chaim permits Answer  I had written this would be pernitted if done for pure motivation but this is true only if the informer spontaneously gives the information but not when the teacher commands it and because of the fear of punishment. In addition it should be possible to correct the misbehavior without demanding the serious sin of lashon harah to know the identity of the one who misbehaved.  

Rav Moshe Sternbuch(Teshuvos v'hanhagos 1:839): Question: A a teacher asks his students to reveal who did something wrong. - is this permitted? Answer: It would seem that the teacher first has to say to his student that the prohibition of lashon harah is very severe. However since this is l'toeles there is no prohibition of lashon harah. (See Chofetz Chaim 4 and the statement of the Alter of Kelm in Marpeh Lashon said in the name of Rav Yisroel Salanter with clear proofs that whatever is for to'eles is not prohibited as lashon harah). In this case it is clearly for the benefit of the chinuch of the student and therefore there is no concern that this is lashon harah. In fact just the opposite because just as there is a prohibition to speak negatively about others - there is an obligation to inform the teacher so that he is able to properly chastise the student and thus it is not considered lashon harah at all.