When the gang arrive at school the day after the rumor hit, T.J. is not wearing a backpack. When the gang walk up to Phil pushing Mikey on the swing, he's wearing a backpack.
When T.J. is wearing his backpack, he only has one strap over his shoulder, but when T.J. tells Mikey that he needs to stand up against rumors, his backpack has two straps.
When the gang find Randall hiding in one of the tires to the Ashleys' clubhouse, T.J.'s backpack is missing. After they drag him out, it's back. After Randall tells the gang that Digger Dave told him the rumor, his backpack is gone for the rest of the episode.
When Menlo stamps the letter to Principal Prickly, it's dated December 8, 1998. When Menlo shows the gang the date on "The Men-Lo Down", the date is December 9, 1998.
The date on "The Men-Lo Down" reads December 9, 1998. However, the show only takes place between September 1997 and June 1998, which means that the episode would've taken place after the gang moved up to fifth grade.