Show up

[Xinfu]

-Students! Show up to :)class!
-Students! Show up in :)class!
-Students! Show up at :)class!

Are they all correct? I think the first two only are correct. Do you agree?

[Rusty]

All three may be used when making a statement or when framing a question, so I'm certain that all three can be heard when using the imperative.
I prefer using 'to' in the imperative.

[Xinfu]

Good answer. Thank you.