In AP2, the scene where Greg (the AP History Guy) beats up a random stranger that stood up for Mike Dame may seem like unnecessary filler. This is because Greg needed to be wearing the shirt before the climax of the movie.
The epic battle in the news room was shot prior to the final scene involving Mike Dame, but after most of the movie had been filmed. Only after stretching their time constraints to the max did the cast realize that Greg was wearing a button down shirt that was not present in the earlier scenes. So they then rushed to find a person willing to play the role of hapless martyr. Dave Deagan was the most readily available person, and so he became a part of the AP History series purely to have his shirt taken away.
And so a shot was crudely thrown together that involved Greg beating up Dave and taking his shirt. It seems awkward in the movie, but there was no other choice. Now you know yet another secret behind the AP History franchise.
This issue is actually referenced in AP3. During the flashbacks where it shows the AP History Guy kidnapping random other students, the AP History Guy is wearing this shirt. Ironically, a similar situation occured during the filming of AP3 where actor Tyler Swett forgot a white shirt on a day of filming. This lead to a frantic search around the school, where they eventually were able to acquire a similar shirt from fellow actor Justin Craig.