The girls have been repeating the same four months for an indefinite period of time, but only remember two beach trips. They have no memory of any beach trips, Christmas parties, or Halloween parties prior to Sensei waking up at the start of the game. That doesn't mean those events didn't happen, it just means the memories of those events in prior loops was lost when Sensei himself reset. Once he wakes up in the classroom, it's as if they're back at the very early part of the school year.
We haven't seen one of those resets in-game. We wake up immediately after one at the start of the game, and that's that. Maya has spent the last two resets bracing for the next one to be another hard reset, but it hasn't happened.
So basically, there are two kinds of resets.
Think of it like your computer. The resets we see in game are the equivalent of rebooting your PC or phone. But when Sensei himself gets rebooted, it's more like the entire device gets formatted and the OS gets reinstalled kind of ordeal.