If you are under 20 while studying, your education is free and the costs will be covered by your municipality. Even at this level the schools are not allowed to ask students to pay fees, even for school excursions. Saying that it is however not unheard of and some schools find a way to get around the problem.
You can also return to gymnasium level study later as a mature student – normally through komvux (see below). In this case, you are entitled to a government grant to help cover your costs and should contact CSN (see university funding) for more details.