Yeah I can imagine Steam being a culprit for a lot of folks that should be doing better at school than they are.
I do quite like the Ubuntu minimal install, since it kind of requires you to at least know what you want on your system, and is very minimalistic about it all, while still maintaining a certain degree of user-friendliness.
The idea of a persistent USB Linux is intriguing, I've never tried it myself aside from a few times with Puppy (which is a great dist as well). However, as for interfaces, I used to be KDE guy until they went south and I migrated to Gnome, which I started to love until they went south. Now I'm on XFCE and I have to say, that so far I'm really, really liking it.
The thing is that the more you will build it yourself, you'll probably come to notice that it will sway away and differentiate from most of those bloated mainstream distros (I'm just guessing right now, as this happened to me) and start to look more and more like something like crunchbang.