The Life of Pi takes the cake as the absolute worst book I have ever read for school or for any other reason, period. The first half of the book was a horrible, boring intro, which stands as my least favorite reading moment of all time. The second half, the beginning of the actual story, was alright at first, but it slowly degraded down to pro-religion symbolic babble. The plot twist at the end was good, but that's about the only positive thing I can say.
Runner-ups include two vapid books about "teenage problems" I was required to read over the summer in 10th grade. They weren't awful books, but it felt the same as the time I was getting a puberty pamphlet in 5th grade.