Harry Potter and the Cave of Freedom

Not that confusing really. Primary/elementary school is mandatory, but you can get expelled.

Yes but typically elementary school students don't have the potential to accidentally magic someone into oblivion (it's proven multiple times you don't need a wand to do magic, it's just extremely helpful)

Plus Hogwarts wasn't the only wizard School if he got expelled from there he could have transferred to another wizard school. Just like if you get expelled from a regular school you can just get transferred to a new school.

The difference being that at least in America you can't expel a student assigned to your district. The worst you can do is suspend them for the rest of the year