Back to school time is when many parents consider buying a new laptop or tablet for their kids. In my experience, it makes sense to look at laptops that are in the $400 range, have good battery life, are durable, and are sized right for a students backpack.
First, there are no suggestions for the class of computers often called "netbooks." Netbooks were quite popular a few years ago when a normal laptop often cost over $600. The Netbooks came along priced in the $350 range. They offered a tiny screen (11-12 inch screen) and great battery life. Where they often were not great were processing power and keyboard. Tablets have replaced most of the sector.
First, if money is not a constraining factor, consider an ultrabook for PC's (like the Asus Zenbook Prime UX31a) or look at the MacBook Air if you're considering Mac.
Here are a few other choices that should be plenty for any student from middle school through college.
Toshiba Satellite P845, $600
Lenovo ThinkPad Edge E430 $550
Dell Inspiron 13z, $500
Update: IF your school is cool with it, a new type of laptop that works well for many students is the Google Chromebook. If your student will have wifi on campus, this laptop is light, has good battery life, and is great at avoiding viruses and issues. In exchange for those benefits, you won't have the ability to run windows programs (like microsoft office)...but online replacements are easy to use (google drive / docs for example).
Samsung Chromebook (Wi-Fi, 11.6-Inch)
After you get it, I suggest getting all the windows updates done, then using an imaging program (acronis true image for example) and make an image backup of the hard drive. That way, when (not if!) the computer gets a virus or corrupted, you can restore it back to exactly how it was the day you imaged it. Other suggestions include replacing internet explorer with Google's Chrome browser (safer, faster, updates without prompting).