Monk and gang are very popular at school, but they bully people. So, they bullied Melvin, but Priscilla doesn't let that happen.
Priscilla gets mad that Monk and his friends are bullying her friend, so she pushes Monk into the locker and suprises Monk and his friends, that she did that.
Treat others the way you would like to be treated.
When Pricilla says "I don't know who Munk is," it is very ironic because Munk and his gang are like the most popular kids in school, so she has to have heard of him.
This is situational irony because who would have expected that Pricilla puts an end to Munk and his gang (and she does it like a champ). This is situational irony because everyone expected to be able to see Munk the next day in the locker, my the was a blizzard and it snowed, and school was out for a week
This is dramatic irony because people are probrably wondering where Munk is, but no one knows that he is in the locker except for pricilla and us the readers.