If you've seen a message pop up confirming that the cheat you typed was correct, that means it's active unless you quit the game (or perma if it was stat altering)
It has nothing to do with the length of the night session
The night can end in 1 of 4 ways:
- time runs out (cheat sets it to 99:99:99)
- stamina/camera battery runs out - this is the most probable cause
- you end the night prematurely by clicking "end" button next to the time - this has the same probability as the one above
- you get caught by either Yuna or Kan
Next time check which of those happened and you'll know what to do next
As for the game itself - it is supposed to be hard at times, but once you get the gist of it it's more like "ah, it makes sense" or "now that I think about it, it was kinda obvious" etc...
The idea is that first time playing you're supposed to fail here and there to learn about stuff and then come back with knowledge on how to proceed.
As for the school part - yeah it sucks balls, iirc it was kinda rushed, but basically those questions are either game lore related or from our real world
Cheers