Let's try a little logic.
You own 10% as do each of the other shareholders, except for the school, who own 60%.
If the club sells their shares to one of the existing members he will own 70% and the other three of you will own 10% each.
The way you've been working up to now is not as a normal group. Normally your fixed costs would be covered by the monthly payment and your variable costs by the hourly charge. However the club has been paying all of the fixed costs and you have been paying less than your share of the fixed costs and over the odds on the hourly rate. This will have to change.
In the normal course of events you'd have availability and costs in proportion to your shareholding. You need to add up your annual fixed costs (insurance, maintenace, hangarage/parking etc), add a little bit for contingency and inflation, divide the answer by 120 and that gives you the fixed costs per share per month.
So how are you planning to apportion it? Does the guy who owns 70% of the aircraft pay 70% of the fixed costs? He may be being philanthropic and helping the group out by investing more dosh but at the end of the day the fixed costs are being divided between fewer people. On the plus side, availability should be better.
How about you come to an agreement to share the fixed costs equally pro-tem with you all trying to sell the surplus shares to bring in more monthly income and reduce the amount you all have to pay? Or reduce the number of shareholders, e.g take it down to 6 shareholders to keep good availability, the three of you stump up some dosh to increase your share from 10% to 16% leaving your philanthropic friend with a half share from which he can then sell two thirds to put you all on an equal footing? There's a lot of ways of doing it, you just have to agree on how.
At the end of the day it's down to finding something that's mutually agreeable. If you can't agree then the way out is to sell the a/c, pay the bills, and split what remains in proportion to shareholding.