How To Ask About My Friends' Subjects?

Sorry,once when I asked one of my US friends what their subjects are, I asked-"What do you study at school?"

I wanted to know what the subjects are that my friend study at school.But my friend seemed to be annoyed for this question and it seemed like an unusual question for him.He answered-"What do you mean,I study many things at my school".Sorry,I forgot what his sctual reply was,so I'm not certain.

So,how can I ask my friend to know the subjects that he studies?

Thanks in advance!!