Weather cancellations are something you just have to put up with. It's a pain for all concerned.
Cancelling lessons is as big a problem for a club/school as it is for a student, unless they've paid upfront of course, but even then, it's better to get people flying than have them waiting on the ground.
If an instructor was off, then unless it's an unexpected thing (e.g. Myself next week. I have a number of funerals to go to and so have had to cancel all my flying next week, it's not easy to get cover at the last moment.) then a booking shouldn't be taken unless someone is available to fly with you.
A/C do also break and sometimes it is just bad luck. Have a chat with the school and tell them of your concerns. If they don't bend over backwards or it continues like this, then demand your money back. You've paid for a service which you haven't recieved.