Is it loshon hara to tell someones rosh yeshiva or rosh kollel that one of there students watches TV?
Answer:
This would be lashon hara (it is both derogatory and liable to cause damage), unless you feel that the Rosh Yeshiva or Rosh Kolel need, for some reason, to know this.
For instance, if there is a rule against TV, and the Rosh Yeshiva is very makpid about it, or there's a risk that the person in question is going to draw others into TV/movie addiction, and so on, then he should be told.
However, if the person is just watching the news (or National Geographic, etc.) on TV, and he doesn't pose a threat to others, then it is lashon hara to inform of this, even if you feel it's wrong for a yeshiva man to be watching TV. If you feel strongly about the matter, you can take it up with the person in question, but not with the Rosh Kolel.