When the school-bus leaves "Fort Springfield", Uter and five other students can't catch up. But seconds later the five other students have disappeared.
During the dinner scene, when Marge gets up to clear Bart's plates and outs her apron on him, the plates she cleared disappear, the bottle and small round plate on Maggie's red sitting chair disappear, and the silverware next to Homer and Marge's plates disappear.
When the man screams about the PTA being disbanded before jumping out the window in a panic, there are different people surrounding him between the first open shot from above and subsequent closer up shots.
Principal Skinner doesn't know that admission to the museum is no longer free until they arrive. In reality, a school visit would've been booked in advance, so he would know, plus it would most likely be paid for beforehand as well.