A lot of the entries don't seem particularly "playground"

Yeah; I detect a lot of Viz-isms; but overall it's a good informal modern take on the territory of Iona and Peter Opie's Lore and Language of Schoolchildren.

This past week was my recess duty week, where I spent a half hour daily on an elementary school playground. My colleagues and I were making sure the children played nicely and didn't use too many b, f and s words. As for playground slang, one of the aides said to a particularly bad boy, "I'ma jack you up," which I didn't fully and deeply understand, but got the main idea. Also, "somebody be trippin' 'bout something".