It’s also possible that “Nova Squadron” is just one of many (as Value Subtracted mentions herein) and that there are many other Nova Squadrons without any issues in the intermittent years between the incidents. It’s possible that, like most things in the Star Trek universe, we only see the times when something interesting/dramatic occurs, not the boring day-to-day.

Said another way, just because you periodically have a rowdy graduating class from primary/secondary education, doesn’t mean you stop having graduating students.