Smaller branch units with as few as 30 students provide education from first to third or first to sixth grade students in geographically remote regions. In bigger cities, school sizes reach over 1 000 pupils. Compulsory education for pupils between 6 and 15 years old is offered in so-called basic schools and is organised as a single structure (without distinction between primary and lower secondary levels). Currently, Slovenian schools must employ a large number of professional support staff, as a share of total staff, and retain them regardless of the size of the school, making the system very costly.