If we're talking through all grades of school, then mine would be The City Boy by Herman Wouk. I read it for the first time in the fifth grade and still like it to this day
I had several! Back in about 3rd grade we read Julie of the Wolves -- one of my all-time favorites. Then there were Islands of the Blue Dolphin and Streams to the River, River to the Sea. In later grades, I loved To Kill A Mockingbird and The Count of Monte Cristo.
Actually the one I remember the best is The Scarlet Letter. We read it our freshman year in high school. The teacher who taught it was a wonderful teacher and made it interesting.