650 is nothing in Sofia, but in smaller towns it's a lot, especially if you own an apartment and don't pay rent, which most people outside of Sofia do. We are the country with the highest home ownership % in the world. 
The quality of teaching is low, but we produce smart kids because the material in the textbooks is of a relatively good quality (math for example). So the kids who want to succeed don't rely on the classroom and study on their own.
The low quality of teaching is why kids are required to pass an exam before being accepted in a Bulgarian university, since the universities can't trust the school's grades to be correct. And these exams are usually several levels of difficulty above anything that is taught in school. A lot of universities still don't accept the "matura" as a valid entry exam because it's perceived as too easy.