It would be more natural to say 'I'm not allowed to watch TV on school nights. My parents think it's bad for my studies.' 'Not allowed' makes it clear that it's a house rule. 'It's' is the pronoun you need for referring back to watching TV. 'Studies' should be in the plural, meaning 'schoolwork'. The word 'study' suggests either a room, or an individual piece of academic research.

Studies not study. You could say my parents think it's bad for my academics, grades, etc.