Complex sentence: Private institutions of higher education are prohibited by the Greek constitution but there exist private institutions that operate as non-accredited Educational Centers (Κέντρα ή Εργαστήρια Ελευθέρων Σπουδών).
Split into simpler sentences: Private institutions of higher education are prohibited by the Greek constitution. However, there exist private institutions that operate as non-accredited Educational Centers (Κέντρα ή Εργαστήρια Ελευθέρων Σπουδών).