This page lists fan theories about the animated film Finding Nemo.
It is about loss
Nemo, when translated from Latin, means "no one." So in this essence, Nemo doesn't actually exist; Marlin just invented him so that he could compensate over the loss of his entire family. Marlin considered this loss his own fault rather than as a freak accident. The sharks helped him to realize that things that seemed like threats at first may not be, so worrying about every single thing in life is futile. The turtles taught Marlin that he isn't expected to have control over everything in his life, so he shouldn't worry about it, and Dory indirectly taught him that he should let go of all of the pain of the past, because it wasn't his fault. Nemo going to school at the end represents Marlin finally coming to terms with his loss.
However, at the beginning when Merlin first attempted to take Nemo to school, the other parents and the teacher all seemed to be able to see Nemo. Were they all just humoring Merlin, seeing him as an eccentric but harmless clown?