In 2001 Greece had an adult literacy rate of 100 %. Education is free and compulsory for all children between the ages of 6 and 15; the remaining years of secondary school are optional and also free. Many Greeks place a high value on education as the key to upward social mobility and a secure job. there are an insufficient number of public universities, and the constitution prohibits private ones, making access to higher education highly competitive. Many students of means attempt to gain an edge by attending privately run educational establishments called phrontisteria, which prepare them for university entrance examinations. Students attend the phrontisteria in addition to high school. Students who fail to gain entry to Greek universities may attend private, unofficial colleges. Many also choose to study abroad, particularly in the United Kingdom and Italy.