Is secondary school compulsory?
Yes, it is. Till the class a child is in at age 16 is finished, kids have to go to school on a daily basis. Till age 18 they have to go to school if they have not finished a curriculum yet.
A child has to follow 12 years of education at least. (If it skips a class, because of being so ahead of it's peers, the skipped class will be counted as a year too)
Children have to go to school the moment they are 5 years old. Most kids start at age 4 though.