Whats the title of this book?

[Mari]

This is going to sound weird but I remember reading this book (fiction) in elementary school, around the late 90's, about this boy who learns that his neighbor or somebody is keeping himself young by putting himself in some sort of like glad food storage container except human size. I don't remember the rest except one character was a grown man, it might even be twins, who repeated like the 5th grade his whole life and so he teaches the 5th grade beacause he took it so much. I know sounds crazy but I remember it so if you know the title please tell me.