1) Please call the teacher as soon as you get off your first class.
2) I will go there once i get off class.
I would accept both these sentences as possible. In AmE we would more likely to use "get off" with "work" or "duty" than "your first class."
"Call as soon as you get off work."
When dealing with school I would tend to say, "Call the teacher as soon as you are finished with your first class."
However, I don't believe that people here would object to the way you have formulated the sentences.