Read your order, it will state the exact requirements for child support to continue to be paid. If you feel that you should no longer be paying support I would suggest that you contact a local attorney and have them file a motion to terminate the child support.
Remember that you pay child support until the court orders otherwise. That means that you are obligated to pay per the court order until the court says to stop regardless of if the child is in school or not.
You must pay child support for as long as your court order states that you must pay. Generally in Texas, the orders say until the child turns 18 or after as long as the child is in primary school (high school.) If so, then if the child quits school but is already 18, then the child support payments stop (for that child.)