Yes you are right dershane is a kind of school. Especially help students on weekends for their exams. Also you are right school mean okul. Also in Turkish dershane means classroom. But for classroom mostly 'sınıf' is used in Turkish

Basically, a cram school that you could go as a secondary school (by secondary i don't mean lycee. I mean an extra school). Every university prep student was enrolled in one a few years ago in addition to its normal school. Since primary cram schools was usually very expensive secondaries was the choice of most of the prep students and they were popular. But now government made an amendment and they all changed it to primary cram schools. Secondaries are banned. And all the secondaries adopted their price and curriculum. As a result primary cram school prices dropped etc These amendments reason actually was not educational concerns. But was rather to obliterate the economy of an organization callled FETÖ ( which you may have heard of). They owned a big percentage of cram schools. And now they are even branded as a terrorist group by the government and accused with attempted coup of 15july. It literally means house of lesson (ders-hane).