If I emailed my teacher:
I've said that I can stay after school to make up the test, but I found out that I have a band rehersal tomorrow. Is it okay for you if I take it on wednesday after school instead?
is the phrase is it OK for you formal in a student-teacher correspondence?
I feel like asking your teacher "is that okay for you?" is not formal and sounds like I'm talking to my friends.
and maybe any grammatical mistakes?