We don't have synonyms for sixth form yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are sixth-form.
(n) :
(UK except Scotland, education) The final two years of secondary education, during which students of about 16 to 18 years of age prepare for their A-level examinations or equivalent qualifications.
(n) :
(UK except Scotland, education) The final two years of secondary education, during which students of about 16 to 18 years of age prepare for their A-level examinations or equivalent qualifications.