「を出る」と「に出る」 wo deru vs. ni deru

クラスを出る。
Kurasu wo deru.
I will leave class.

クラスに出る。
Kurasu ni deru.
I will attend class.

Basically, the particle depends on the direction in which you're leaving. If you're leaving something, use を; if you're leaving for something, use に.