Alright, class, we have 39 students and 40 seats.
That one dyslexic kid thinking he’s Superman:
Dyslexic Superman
Explanation
The humor comes from the idea that a dyslexic kid might misread "39 students and 40 seats" as the opposite, leading him to think there's an extra student, and he's it. It's funny because it plays on the common issue of dyslexia.