I would personally shy away from 15.4" laptops. I've never liked them and I always find them an odd standard that somehow caught on. To me, a normal laptop should have something between an 11.1" and a 13.3" screen and less than 4.5 pounds. Here's a small selection I like (and I find balanced) from NewEgg.
Lenovo ThinkPad SL400 (14.1" screen, C2D T6570 1.8GHz, 2GB DDR2, 250GB drive, GMA4500M HD, Vista Business)
Toshiba Satellite Pro S300M (14.1" screen, C2D P8600 2.4GHz, 2GB DDR2, 160GB drive, GMA4500M HD, Vista Business w/ XP Pro downgrade)
Lenovo ThinkPad SL300 (13.3" screen, C2D P8400 2.26GHz, 2GB DDR2, 160GB drive, GMA4500M HD, Vista Business)
Toshiba Satellite U405 (13.3" screen, Turion X2 RM-74, 4GB DDR2, 320GB drive, Radeon 3100, Vista Home Premium 64-bit)
Then there's also netbooks, which should meet most school needs. These laptops are smaller, lighter, and cheaper than larger laptops, but are not particularly fast. Plenty for using the web, instant messaging, watching normal videos (not high definition though), and using normal programs like Word, Excel, Powerpoint, and the like. Basically, it's a laptop for 95% of what you normally do. And yes, it can play Runescape full-screen. I can't say much about the performance on newer netbooks, but the performance on my EeePC 900 (with a 900MHz Celeron M and using GMA900) was good enough to play, but I still much prefer my desktop. Here's a small selection of netbooks that I think are good for the price.
Asus EeePC 900HA XP (8.9" screen, Atom N270 1.6GHz, 1GB DDR2, 160GB drive, WinXP Home)
Asus EeePC 1000HA (10" screen, Atom N270 1.6GHz, 1GB DDR2, 160GB drive, WInXP)
Samsung NC10 (10.2" screen, Atom N270 1.6GHz, 1GB DDR2, 160GB drive, WinXP)
The final laptop I'd like you to consider is actually a convertable laptop. This one has a screen that will swivel on its hinge and make it into a tablet. The price is VERY good for what you'd get, as well.
Lenovo ThinkPad X61 (12.1" screen, C2D L7500, 1GB DDR2, 160GB drive, GMA X3100, Vista Business)