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.
But! Being able to find clipart that is more than the standard stock clipart that you receive from your Microsoft package is important. And the best is the stuff you can find that is free, of course! But how do you find clipart that you don't have to pay for?
Do take the time to write a personality profile articulating the characteristics of your cartoon brand mascot. Write his life story. Tell your customers all about him. Flesh out his personality, and be sure he behaves consistently with who he/she is.