Grammar Confusion

The teacher wanted to find out how much knowledge her new students had retained over the summer vacation.

She wrote the following phrase on the blackboard: "I ain't had no fun since school started."  

Then she turned to the class and asked, "How would I correct this?"

Before any of the other children could think of the proper sentence structure, a sandy haired boy in the first row raised his hand and shouted, "You should get a hobby and quit school!"