I have the x2 visa, how can I leave china and come back again?

Usually, a X2 is a single entry type, so you cannot leave China and then back again, unless you apply another one when you out of China.

Or you can consult the school whether you can change your X2 to a resident permit. It is heard that some school/university can do that. If the X2 can be changed into a resident permit, you can use the permit to enter and exit as many times as you want.