The headmaster ( of a boarding school) punishes the two servants by denying them any food the next day. To stay full, Sara ( one of the servants) suggests that they eat a feast that night. The two girls then pretend to have a banquet in the room.
what does "to stay full" mean in this context?
They don't actually 'stay full' (remain in a state of having eaten enough), but they stock up before the enforced fast. The idea is that the more they eat, the longer they can last without food. (This works in the long term - most animals eat more at one time of the year than another, and live off thier fat reserves when food is scarce), but I don't know whether it would work as well over a two-day period.