It's just a nomenclature as assigned by the society over period. But no all the colleges are said to be college, in Abroad there are some colleges or universities which are said to be the school like School of Management etc.

In India also, we have institutions Delhi School of Economics in New-Delhi which is a grad-school.

So don't bother about the names and how they are called. Just keep studying.

I hope this answer helps. All the very best for your future endeavors!