Hi, sorry, I was asleep/working/playing a brass instrument ... I'm back now. Sounds like you've tried it all. You should see the shit state-of-repair of instruments Mr Limey is sent to teach on
Leave it in a bath of warm water and dishwashing soap for a good few hours, like Ali says, but it seems you've done that. If the school is going to fleece you for repairs/a new instrument it might be worth seeking out a repair shop and paying them to do it (they have Waysİ), which'd probably work out cheaper.