Question

I have seen a classmate tear pages from book in the library.what should i do now?

Answer 1

as a first step,you should tell him not to do so as it is not his property.  as he is your classmate you would be knowing his behaviour, and so you could advise him the way ,you prefer,that fits the situation. if he is not hearing you it is better you inform  the librarian about it, as you are not the one who should take the action against him

Answer 2

The best you can do is to tell the labrerian about it.
As you were also present there you may get involved about it if you don't inform the librarian.