I think it must be based on the Marauders Map. Like, 4 boys randomly finding a piece of parchment, then taking it along for rough work (they keep it on the top of their equipment). And when three of them are sitting and talking about exams. The fourth one joins them and keeps his wand on the pile of equipment commenting of exams, saying"I solemnly swear I am up to no good", then they find out about the magic of the map. Sometimes they sneak into the kitchen etc. Then they find out more about the map, about the creators Moony, Wormtail, Padfoot and Prongs. One of them denies to use the map as 'he thinks' that the previous owners suffered a great deal cause they used the map. Then the other 3 notice that some unknown people have been entering the castle and decide to warn and inform their 4th friend. Likewise, they save the castle from a pack of werewolves when Dumbledore and Professor McGonagall were away for ministry work.
Hope you like it :)