With есть you confirm the existence of the free time (and when that is is less important), without есть you imply you have free time and the emphasis of the sentence becomes on when you have free time (after classes). I think both sentences should be correct.
I almost totally agree with this comment. Why almost? Because its first part is about the cases when it is better not to use 'есть'. So using есть is not always better.
Anyway I would like to give this man 10 lingots because his comment is the best in the thread.