Troll

There are a lot of things that have made me laugh in this life that maybe weren't all that funny. The word "chicken" for example.
The sense of humour that appeals to a child isn't always congruent with what your typical adult feels is funny. However, I must admit that, when reading Harry Potter the suggestion of somebody getting a letter grade of T (a failing grade) is quite funny, merely because the T stands for Troll. There is something absurdely hilarious about an institution such as a school awarding students a grade that essentially does nothing but insult them.

Billy: Outstanding
Tom: Exceeds Expectations
Greg: Poor
Jenny: Outstanding
Hubert: Troll