1. We are at my cousin's school.

Question

2. We are in my cousin's school.
[Can we use both prepositions?]

3. He plays his violin on stage.
4. He plays his violin on the stage.
[Are both correct? Which one is commonly used?]

5. I move in my seat.
6. I move on my seat.
[Which preposition do we have to use? Are both okay?]

Answer 1

All are acceptable.