Personally I recommend the Casecrown iPad cases: they're practical and sturdy and give enough protection. The purple one also looks cute for the girls, and cool for the boys.
This cover will give the iPad the right position for typing, gaming and showing off pictures (vertically or in portrait mode).
A bit more expensive is the Speck Products PixelSkin HD Rubberized Wrap Case for iPad. The PixelSkin wrap gives a bit more proctection than the Cascrown does, because when closed the flap closes (sucks) itself close.
Like the smart cover (the Apple invention specifically made for the iPad) the screen protector folds round to form a stand. There are people who like this, and people who say it doesn't work well on the lap. If you're one to use your iPad on your lap, you may want to go with another iPad case.
The Otterbox iPad Defender Case has very mixed reviews. I looked at the negative ones and they look very suspect to me: users who wrote only one review: that one. And they all sound very much alike.
Positive reviews of the product on the other hand are usually by amazon reviewers who have given at least a few more, but often many more, reviews. In short: I'd recommend this product, despite the negative reviews.
This looks to me like a bit more high tech protection solution: the stand can be set in two angles: one for typing, another for viewing. The inner protection includes memory foam protected by a silicone skin.
If your teenager is at all like my brothers were - throwing their bags around - this extra protection will come in handy. And the case is designed to work with Apple accessories like the dock, so the case never has to come off.
I had to include at least one cool case in this overview, didn't I? We're talking teenagers after all.
I do think the Angelcase Black Ipad Case with it's monkey faces fits the bill. It's a simple silicone snap on face, technically like the silicone cases your teen will be used to for their phone. It offers only the most basic protection, but will still prevent the corners from breaking, for instance.