I'd look around thrift shops for a used mountain bike from the '80s or '90s, like a Specialized Hard Rock, Bridgestone MB3 or the equivalent. They're decent bikes, reliable, use common components and are widely available here (Reno, NV) for much less than $100. A tire swap and whatever rackage you need will set you up. I bought my wife a Hard Rock three years ago with an Avocet computer ($25) and Blackburn rack ($40), plus good road tires, for $40 total. No problems; works great.
For commuting and school, especially in an urban area, you don't want a flashy-looking bike. The theft rate is staggering. One of my neighbors' daughters had four bikes stolen in five years.