Do I still need to pay child support for my child even thou he only goes to school for 3 hours a week and he is over 18?

What does the court order stipulate ?

Where? If you are in the US it varies by State. And, yes, the Divorce Decree (or Court Order) is binding until/unless it's changed by another Court Order.

Are they taking classes to be graduated from High School? Or is this some form of continuing education ?

As was mentioned... Where is this... As in Country and state... and what does the divorce Decree state... because in many places it ends promptly at 18, but not all. These are two VERY important questions that need answered.