There is no hard and fast rule about how much gap is allowable between employment. Some people like to take time off in between while others want to start right away. There is always a potential for overlap between jobs once your resignation has been submitted.

A lot of contracts require 2 weeks notice to get your vacation and other applicable payouts.

Attempting to time your resignation in case your class gets canceled is probably pointless. Most airlines won't let you cancel a resignation. Once it's sent, it's done.

Your mileage may vary.