This achievement simply requires you to do the Snack Boss minigame a certain number of times. I don't know the exact number of times, but it shouldn't be a lot. Probably around 10.
There are two times you need to do the Snack Boss minigame to progress the story, once on day 3 and once on day 4. In addition, you need to do it once on day 1 and 2 each to avoid demerit points. You can also get 2 bonus merit points by doing the minigame 5 times on a single day - in addition to any merit points you get from teacher orders!
So try to get at least one day with 5 rounds of the Snack Boss minigame, in addition to doing it at least one per day, aiming for at least 10 total.
The achievement unlocks when you complete the game.