Pupils worshipped outside and stopped being secret (12)

'stopped being secret' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?

'pupils worshipped outside' is the wordplay.
'pupils' becomes 'class' (I've seen this before).
'worshipped' becomes 'deified' (I can't justify this - if you can you should give a lot more credence to this answer).
'outside' means one lot of letters goes inside another (some letters go outside others).
'class' inserted inside 'deified' is 'DECLASSIFIED'.

'and' acts as a link.

Can you help me to learn more?