Spag Union Military Academy is an all-boys military school that Greg's dad tried to send him to if he did not behave well. Boys could attend Spag Union once they reached the middle school age. Greg did not go to Spag Union because during Seth Snella's half-birthday party, Greg saved his dad from having to try to make Seth laugh by doing funny stuff by trying to get Seth's blue blanket back, ending up hanging on the tree and his pants falling down in the process. That accident prevented his dad from having to act in front of Seth, and his dad payed him back with not forcing him to go to Spag Union. It is briefly seen in The Last Straw and mentioned in Dog Days, including the Dog Days movie.
In the movie, Rodrick finds a Spag Union DVD package in the mail a day after the work lie backfired. While he and Greg are watching it, it makes them worried they would end up there. The film also showed a student who used to play video games every day, but when he got sent there he started reading math books. Another student used to lie that he would be studying, but he would hang out with his friends. He now has a tracking device so he can't sneak out. After the tape, Rodrick says to Greg that he should just join the Wilderness Explorers to prove to his dad he won't need Spag Union. Frank does not send Greg to Spag Union, after learning the truth that his neighbour Mr. Warren made fun of him and after he accused Greg of burning his tent.