In addition to the asthetics, you'll want to consider the format the art is provided in. Raster, or pixelized images do not give as good of quality in print projects as vector, or eps images. If you want to change the colors to match your school colors, and if the images are going to appear large (on a banner or sign), be sure to look for vector eps images. You will need some type of vector based illustration program to change the colors. Coreldraw and Illustrator are the two most commonly used programs. If you don't have one of these programs, check with the Web site provider of the clipart. In many cases, they are glad to make color modifications on your behalf for a modest fee.
2. Delete or add objects - Clipart often consists of many pieces joined to make one object. Because of this, you may come across a piece of clipart that's just too busy. But all is not lost. If you only want parts of the clipart, you can simply delete the parts you don't want.
Inkscape is also widely used by a lot of people, many of whom have created tutorials showing you how to use the software, and that makes it even easier to download a piece of vector clip art and use it in your project.