Let's not cloud the issue. They're taking the p!ss.
Leave as soon as you can and do everything within your legal power to make sure that they lose money from your decision.
Get everyone to walk out on a beautiful day.
Allow students to find out you are now working at a reasonable school.
Any of the work that you did on the groundschool is probably yours and not theirs as you weren't in their employ whilst devising it.
Whoever they are should be ashamed/fined/shut down or all three.
Yes, I've been there, yes I've done it, no, not for long.