Ever since witches and wizards existed, there has been a bunch who weren't like the rest. Had unique powers of some sort. Think of seers and metamorphmagi, just born with some kind of odd talent. Some were just little things, but others were serious and needed to be contained.
And so Octela Academy was created. It was for the strange ones, and so they could have be educated without feeling like an odd one out. Students come from all over the world to this school, when they're just 'not right'.
As time went on, Ocleta Academy became known as one of the best wizarding schools in the world. Now, rich families convince staff to let their child attend, even if they're normal. Despite this, the population will always be at least 2/3 weird and 1/3 normal. And the same goes for the professors.