I am looking for a manga or anime where the MC is funny/lively/loud or anything of the sort turns dark and serious because something happened. It would be great the anime took place in school, but not necessary.

A great example is Shinichi from Parasyte.

I have not read Parasyte so I can't exactly say that these fit but give them a try.
Immortal Regis might qualify if you count its sequel
Vinland Saga a couple chapters in it shows the protagonist young and cheerful