As a senior student myself just starting my last year of high school, I have been bringing my 13" MBP to school since last March with nary an issue.
As long as you keep an eye on it AT ALL TIMES and use it when necessary (you mentioned the library and your online classes so thats not bad, I use mine in nearly every subject, why write when I can type?
) then you shouldn't have an issue. Oh and having a large group of people you know does help
*no attempts of pinching, knock on wood
* then again being tall, senior and overshadowing helps too
Word of caution, be wary of carrying object that have ANY kind of point on it i.e power supplies or anything. I left my power supply in with my MBP and it came out with a nasty lot of deep scratches
but yeah, books and such should be no issue.
You should be fine, I had your concerns when I brought mine to school. Nothing to worry about
for as long as your vigilant and sociable. Yeah.
Goodluck
EDIT: The computers in my school here in Aus are just as crappy. Still on XP and using dog slow Celeron/Pentium 4 crap. Gotta love public education. Half an hour to start Word '07
tried and tested.