Setting up a mixed model

Dear all,

I have data on 400 school students. For each students I have three dependent variables, all continuous.

In addition, I got several independent variables, such as the student age, gender, etc,...

Some students come from the same class. In the file I have data on the teacher: Age, gender, etc,...

I need to find which independent variables relate to the dependent ones.

I naturally thought of a mixed model, with the class being the random effect.

What I am not sure about, is how to treat the teacher related variables, such as teacher age. Do I add them as fixed effects, or random effects ?

Assume that my independent variables are S1, S2, S3, T1, T2, T3, Class, where S are student related variables and T are teacher. I will define class to be random effect. How should my model look like ?

Thank you !