More Sideways Arithmetic From Wayside School by Louis Sachar

From the Newbery award-winning author of HolesEverything in Mrs. Jewls's class has gone topsy-turvy...again. The report cards are scrambled, the pop quiz won't pop, and watermelons are falling out the windows. On top of all that, the boys are upset because Allison has announced that only two of them will be invited to her birthday party, since if more than two boys come, they'll all start acting silly. You never know how things will add up at Wayside School.