I've advocated before that baseline prestige should be a soft floor; lets say you take over a B- school. If your performance is bad enough, your team can fall below a B-; if that happens anytime after your first four seasons, you are fired (there is a 4 year grace period so you can have all of your own recruits for at least one season).
But once you fall below, it becomes easy to bounce back to that level. So if you take over a B- school that slipped down to a C+, it takes pretty minimal success to get back to B- (i.e. computer is coded to give you a boost back if you're close).