Safety Patrol
Safety Patrol is a service ran by Westmore Middle School. Its purpose, as seen in the movie, is to act as the "police" of the school. Various tasks Safety Patrol members have to do include: walking pre-schoolers to their day-care and enforcing the laws of the school. Safety Patrol members have a lot of privileges - free hot cocoa, missing classes, and protection from bullies. It states in the first book that anyone who picked on a Safety Patrol member would be suspended.
Greg and Rowley join Safety Patrol because of these benefits. Greg, however, abuses Safety Patrol by making the pre-schoolers that he was walking to their day-care jump into a large hole. This was because Greg thought that the teenagers from Halloween were about to drive past, so he was trying to hide from them. This plan backfired, however, as not only did he mistake a man in a pickup for the teenagers, he was also spotted by Mrs. Irvine, who mistook him for Rowley (Greg was wearing Rowley's coat). This resulted in Rowley being suspended from Safety Patrol. A while later, Mr. Winsky finds out the truth about what had happened, and Greg was suspended instead. Rowley was promoted to Team Captain.
References
Diary of a Wimpy Kid, 20th Century Fox (2010)
Diary of a Wimpy Kid by Jeff Kinney, first published by Amulet Books (April 2007)