The most likely reason that Millie decides to put snow in Kacey's backpack, purse, and locker could be revenge or a prank. It is possible that Millie might harbor negative feelings towards Kacey and wants to inflict some form of harm or embarrassment on her, leading her to carry out this act. However, without further information, it is difficult to determine the exact reason for Millie's actions.
The most likely reason that Millie decides to put snow in Kacey's backpack, purse, and locker is to prank her.
