This is what a group, of a murder/shrewd, or Slytherins do.

They either

1. Are plotting revenge on someone who hurt their friend(aka their hufflepuff)

2. Are plotting on how best to sneak off to Hogsmeade

3. Are plotting on how best to win the next Quidditch game

They are plotting no matter what if it's a group of 5 or more Slytherins