Welcome to the forum, yungchunli,
The 'it' is fine, though not essential. but not much else is natural. Few teachers would describe their job as lecturing in the classroom; even if they did, 'the' is necessary. If you are saying what job a schoolteacher does, then we need a comma after 'schoolteacher'. If you are talking about one particular teacher whose job is to lecture rather than teach, then you could say:
He is a schoolteacher whose job (it) is to lecture in classrooms/the classroom.
That sounds a little strange though - if his job is lecturing, then he is now a lecturer.
Interested in Language