Bar Exam is the third part of the 11th episode of season 3 in Dexter's Laboratory. In this episode, Dexter's parents find out that their son has been rationalizing to not participate in P.E.. Dexter must pass the exam in order to move up a few grades.
When Dexter's parents discover that Dexter is failing P.E. (Mostly because he is making false excuses so he wouldn't have to do exercise) Dexter is offered the chance to move up a couple of grades, on the condition that he must perform 10 simple pull ups in order to pass Gym and move up a few grades. Later in his lab Dee Dee came in and told him what's he doing and Dexter told her that he made an invention that will make him pass gym, but Dee Dee told him that was cheating and started training. Then Dexter did a pull-up, but Dee Dee told him he needs to do 9 more to pass his exam. The next day at school, Dexter "passed" his exam and was now to move up a few grades, but in the end it reveals he was in the same class with Dee Dee he told her it must be a mistake, but Dee Dee said "There ain't no mistake.".Dexter tried to escape and states he cheated, but it was too late.
The concept of Dexter having a excuses for P.E. goes way back in season 1 episode "Dexter Dodgeball " where he tried to give the gym teacher a excuses to let him miss gym forever, only to learn that the regular gym teacher is sick and new sub ripped up the excuses, forcing him to stay in gym.