English
posted by John .
After school, I go to an afterschool class to study math.
(Is this a correct sentence? Do you ahve better expressions?)

Since you've used "after school" twice, this would be better:
After school I go to a math class.
or:
I take an afterschool math class. 
Thank you.
Then can we say:
My favorite class is a math class. 
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.