There are simply things that a PC can do that a Tablet can't. If you want to play the really intense PC games, obviously a tablet won't cut it. If you plan on using programs that really require a mouse, a tablet probably won't work well for you either. If you plan on creating, editing PDFs, Excel (you can make Excel sheets on the Xoom, but it is cumbersome), Powerpoints (also cumbersome on the Xoom) you would want a PC.
In short, a tablet can't replace a PC yet. That being said, having a tablet is extremely mobile and does replace a lot of things that I would normally use a PC for. The mobility is the biggest attraction for me because even though I have a 4-5lb Tablet PC laptop, it is still heavy enough that I end up not taking it to class after awhile. And the battery life of most tablets blows most laptops out of the water. So I can carry only the Xoom around and be able to use it for the whole day, but have to carry the charger cable with me if I am using my laptop for anything over 2 hours.
But in the end, I don't think a tablet will suffice as your only option for school. I'd get an inexpensive Netbook (as long as you aren't a hardcore gamer) for things you simply need a PC for and a tablet (if you have money left over) for everything else. Truthfully, a tablet is luxury item. It can be a very useful luxury item, but for the same price you can get something that does more than a tablet.