The Thai education system has three main levels: pre-school which is optional from age 3; primary school which is compulsory from age 6; and secondary school which is split into lower and higher levels, with the lower level compulsory until around age 15. Schools are generally open to all students, though some more competitive schools select students based on entrance exams. The school year runs from May to March, with holidays in March/April and October. Public education is free, though additional costs exist for uniforms and materials; private or international schools have much higher annual fees from $6,000 to $20,000 USD.