Question

After school, I go to an afterschool class to study math.

(Is this a correct sentence? Do you ahve better expressions?)

Answer 1

Since you've used "after school" twice, this would be better:

After school I go to a math class.

or:

I take an after-school math class.

Answer 2

Thank you.
Then can we say:
My favorite class is a math class.

Answer 3

Yes, we can say that.

However, as I said before, it's more common to say: My favorite class is math class.

However, if you are taking two math classes, it's more accurate to say which math class is your favorite.

Example: My favorite class is algebra.