We don't have synonyms for s level yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are a level, o level and y level.
(n) :
(UK, historical, education) A public examination taken by the most able A-level students and typically used to support university applications, particularly to Oxbridge.
(n) :
(UK, historical, education) A public examination taken by the most able A-level students and typically used to support university applications, particularly to Oxbridge.