For each Magazine, you'll need to have raised your Stats to be able to collect them.
Find the first stack in the teacher's lounge on the second floor at school by talking to Miss Dewitt and having high enough intelligence to answer her question.
The second you can get at the basketball courts but you'll need to have the dexterity to slip away from Dexter. 
And the third you can get from Kevin in the Cafeteria if your charisma is high enough. 
Back at art class you and Mia can go ahead and make your collages which will end class for the day.