In high school you start preparing for panellinies (similar to the ACT/SAT for college) about 1-2 years in advance. Panellinies is a series of 4-5 tests that you're required to take to get into college, but the path you choose to study in high school determines the type of subjects on your tests for Panellinies. Your overall score on panellinies will determine which college and ultimately what career you have. A medical school will say "we only accept a panellinies score of 20" and a school to become a police office might have a score of 15. After you receive your score, in August you will check to see what scores each college is requiring for admittance into the school - each year this score changes because it's based on how everyone does in Greece that year. The max score is 20.
What's different about Greece than other countries like the USA, AUS, or CAN is that you don't need an undergraduate degree to become a doctor or a lawyer. Once you graduate high school, and if your panellinies score is high enough, you can go straight to medical school or law school.
Can international students go to college in Greece? Yes, but most colleges will require you to speak fluent Greek, but there are some colleges that are English speaking (e.g. American College of Thessaloniki).
Ask away with questions. My SO, took panellinies, went to college in Greece, and graduated from a college there. She graduated a few years ago, so she's fairly up-to-date on how the system works.
Also, something random, you can legally drop out of school at 15 in Greece and not finish high school.