Panel 1: Gordon tells Mike that he doesn't like the idea of watching whatever it is he's about to see. Before he can say that they should leave (and perhaps alert the chaperones), Mike tells him to get down.
Panel 2: When Mike asks Gordon if he recognizes the two older boys that just showed up, all Gordon knows is that they're not from their school.
Panel 3: Mike then notices that they're passing around a couple of bottles. Gordon sets up the punchline by asking Mike what he thinks is in them.
Panel 4: Mike doesn't know exactly what is in the bottles but it probably ain't cream soda.
Summary: We're about to see Mike's idiocy on full display tomorrow when he prevents Gordon from doing the right thing....said right thing being "getting authority figures to chase away the high school kids and also tell Peter's parents to come pick him up for being a moron."