Education in Poland is free and compulsory for all children between the ages of 7 and 15, although growing financial and space constraints sometimes require large classes and double shifts for students within the school day. On completion of the eight-year elementary school program, nearly all children enter the secondary school system. About one-fourth of these pupils attend four-year general secondary schools that prepare them for college or university entrance. The rest attend vocational and technical schools, which provide five-year courses combining vocational and general education, or basic vocational schools, which provide three-year courses. There are also a number of private schools.