According to PG, "There are three variants of procrastination, depending on what you do instead of working on something: you could work on (a) nothing, (b) something less important, or (c) something more important. ... The most dangerous form of procrastination is unacknowledged type-B procrastination, because it doesn't feel like procrastination. You're "getting things done." Just the wrong things."
I would guess coding something fun instead of doing school work falls into (b) for most people who do it. Reasons: 1) Being fun makes it easier to do. 2) If it has sufficient complexity over the school work one can rationalize to themselves that working on it is more important (for learning) than getting good grades.
full link: http://paulgraham.com/procrastination.html