Not Quite Remembering Woodstock
This was a scene in the episode, Hey Arnold! Grandpa Phil did not graduate school because he quit his studies after the Great Depression forced him to work to support his family. The main character tries to convince Grandpa to go back to school and finish his studies because he still has a lot of brain cells.
Source: Reddit
However, Grandpa reveals to him that he has few brain cells left after Woodstock. Therefore, he might not be able to grasp anything in class. This joke was too mature for the kids to get it then, but you can now see the funny drug reference.