Pixlr Express Plus for iPad is a free app that provides intuitive tools for cropping your pictures, fixing the light balance, removing red eyes, and changing the contrast of your pictures. Pixlr Express Plus also provides some Instagram-like filters that you can apply to your images. In the video below the community manager for Pixlr demonstrates how to take an ordinary picture and turn into a work of art on your iPad. The video is about six minutes long, but you can probably skip the first minute and still get everything you need to know.
I recommend Pixlr’s tools because they don’t require you to create an account. And if your school uses a mix of iPads, laptops, and Android devices Pixlr has something for everyone.