The explosion in laptop purchases has followed the improvement of our wireless access. When deciding to purchase a laptop for a student there are several factors to consider: performance, battery life, screen size and weight.
Performance - while a netbook might look useful and inexpensive, most cpu's are underpowered. Look for an i series Intel cpu or equivalent in AMD.
Battery Life - many of the inexpensive models in Future Shop or Best Buy are good machines but suffer in this area. You may get three hours at best. Look for an eight cell battery or higher.
Screen Size - this is personal preference. I'd suggest a minimum of 12 inches. Going with a 17 may do if you are using the laptop as a desktop replacement but is tough to lug around.
Weight - relates somewhat to screen size. A netbook may come in at three pounds. A 14 inch laptop can be five pounds and is reasonable to transport to school and back.
Suggestions (these are from personal experience and observation):
Lenovo Thinkpad X120e - a light weight star. No optical drive but is it really necessary for school? You can always buy an external one.
Macbook Pro 13 inch - yes they are pricey but offer tremendous performance and battery life along with a stunning visual display. They very popular at our school.
Asus UL20FT-B1 - a sleek and quick performer. Asus offers a standard two year warranty, making it attractive.
Acer Timeline X series - a light alternative from the not always top-of-the-list Acer. Also available at Costco and Staples.
Final thoughts: consider an extended warranty. Rule of thumb, the more valuable the asset, the more reason to buy the warranty. Although I did buy one with my daughter's $700 Sony and took it in six times! Students can be tough on their laptops. Also, buy a good knapsack intended for use with a laptop.
Good luck!