Bee's knees - idiom still used fairly frequently in BE, meaning 'very good/best'
Not cricket - idiom rather dated even in BE, meaning 'bending or breaking not the rules but the spirit of fair play in any context [not just cricket]'. It's still used, but in a spirit of gentle mockery of someone's old-fashioned fairness: Old George wouldn't read a fellow's diary, even if it was lying open on the desk - it wouldn't be cricket. There are many variants of 'not cricket': just not cricket, not quite cricket, hardly cricket... but always 'cricket'.
- For Teachers