I am a new PB owner, and I bought my Booq Powersleeve for these exact reasons. I wanted something I could carry my laptop in alone if I was going to visit a friend or to the library, but I'm also a graduate student and teaching associate--so I wanted something I could fit into my schoolbag as well.
For me, it sure does the trick. My schoolbag is smaller than a regular backpack, and it has just a shoulder strap to it. My Powersleeve (12") fits into the bag leaving enough room for my class binder and handouts/papers to grade/coursepack. I probably couldn't fit more than an additional textbook in there in addition to everything else I've just mentioned. In the back is the large zipper pocket. I haven't tried to fit 8.5x11" paper in there yet--I'd be happy to try if it's something you wanted to know about--but I carry some smaller scrap paper and a pen. In the front pocket, I fit my AC adapter (and sometimes keys too). That's all I take with me.
Since I don't know what size schoolbag or Powersleeve you're looking at (edit: Now I do, but only because you posted while I was composing this message!), I'll go ahead and mention that my husband has the 15" Powersleeve. He's very happy with it and can fit it into a standard backpack with books when he wants to. The most noticeable instance that comes to mind is when he carried it this way when we flew to San Francisco, so that the laptop bag would be hidden while we were waiting in the airport.
Hope this helps!
