Get a 13.3-14" ultrabook which weighs less than 4 lbs (or around 1.5 kg), an 11.6" screen is too small for doing work imo and anything bigger is a pain to carry in a bag.
You want something with a higher screen resolution but the standard 1366/1378 x 768 on these is also fine especially considering the price range (should not imo spend more than $600 or $700 on a school laptop). You can get away with spending less but I do recommend getting an ultrabook due to the portability reason and since they're a new version of a laptop, they are slightly more expensive at the moment.
Anything with a 3rd gen processor is fine. i5 3rd gen processor would be ideal since i7 is more than you need and i3 can be a bit weak with certain applications but even an i3 processor should be okay. The standard 4 GB ram is also okay but the ram should preferably be upgradable and it should have something called an SSD which should also be upgradable. An SSD is a type of hard drive without any moving parts. Most budget ultrabooks come with a 24 - 32 GB SSD and a 500 GB - 1 TB standard hard drive. The SSD is better to have because it generates less heat, uses less battery, and fails less often than the standard HDD. If it's upgradable then you can take out the computer's hard drive, turn it into an external drive and replace it with a 256 GB SSD which should cost around $100-150 to buy. You don't really need to worry too much about graphics cards, it will probably have an Intel GMA or Intel HD 4000...neither are great but they're fine for video or normal tasks and you aren't gonna be doing any heavy gaming. A touch screen is a nice option to have as well and I prefer a computer with a matte screen instead of a glossy screen.
A good value ultrabook is the Asus Zenbook UX32A (there are different models in this one, some have an i3 processor, some have an i5, etc). If you can spend a bit more than the Asus Zenbook UX31A is also good. A couple other affordable ones are models of the Asus Vivobook S300 and S400. The Lenovo u310s and u410s have some issues but there's a new model released, the u440 is already in the US at BB for $699 and it has a new 4th gen i5 Haswell processor. HP Envy 4 (touch screen version if possible) is also a decent budget ultrabook.