Upper secondary school in Finland. Between the mandatory 9-year elementary school and further university studies. Not really the Finnish equivalent of high school, although often described as such. A similar school system can be found in Sweden.

Lukio is usually completed in 3 or 4 years, and often attended between ages 16 and 20. There aren't any strict classes, so one can pick courses from any class' selection. At least 75 of these courses must be passed in order to graduate. Some subjects and courses are mandatory, others the student can freely choose from.

Graduation also requires passing final exams, which take place twice every year. One doesn't have to take all of them at once. The finals a student must intend include finnish, swedish (the 2nd national language), mathematics, 3rd language (usually english) and "real". The last one is a group of other subjects like biology, physics and history. One can leave either the math or the "real" out if they wish.
Once a lukio student has graduated, he/she can apply into a university or a similar institute.

I, along with many others born in 1979, took part in an experiment where we only had a miniscule amount of mandatory courses and could build our curriculum very freely. This was supposed to become the new national system, but they found out it didn't work. Because of the excess freedom, lazy students (like myself :) easily had trouble falling behind.