That’s really not true. Here are counter examples in each book:

1. The stone was safe with his help, and Quirrel probably would’ve gotten it eventually w/ Voldemort’s help
2. The school would’ve been closed and Voldemort would’ve returned as Tom Riddle’s memory
3. Buckbeak would’ve died, and that’s about it
4. You win this one; Harry would’ve lost if he didn’t cheat and Barty would’ve needed another plan
5. Not sure if there’s a way to get Harry’s prophecy without him, so maybe you win this too?
6. He basically does what Dumbledore wants the whole time

The real issue is that the adults all suck. Surely an adult could design challenges a few first years couldn’t crack, or figure out where the entry to the chamber of secrets is. If Harry followed their rules, things could’ve been much worse because the adults are largely incompetent.