My school has about 3-5 different types of computer models. They use a different image for each model, but use the same image throughout all computers of the model.
The school may be 2 years behind on the OS before it upgrades, but it saves the techs the headache of trying to figure problems out with the new OS. When they bought brand new systems for my middle school about 6 years ago, I always wondered why they would downgrade it to 2k Pro. The next year they got 1 computer lab with Dells that had XP Pro, but they kept the rest 2k Pro. Heck, even my freshman year in HS, they still had over half the systems with 2k Pro. It wasn't until Sophomore year that they finally got it all to XP Pro, I think. Now I'm a Senior, I doubt they will have anything new with Vista or I'll be really surprised.