We use "Classroom Faces" online. http://www.classroomfaces.com/
The kids draw their own face with a black sharpie on a template they have and the company makes the front of the shirt with all the kids on it. It turns out so cute! They are very reasonable and we have had good luck with the company. They are very good to work with. We get our school colors. (purple with white lettering). We have found that the white lettering washes better over time rather than a white t-shirt with colored lettering.
Please let me know if you have any other questions! They have everything you need to know online but I can clarify anything for you!