I can't really tell you exactly how they were made... as I personally wasn't involve in making them. Each child had a 4x4 or 6x6 canvas, color washed it, lined crayons along the top and melted them, used letter stamps to spell out a message like - "Reach High", "Think Big," etc. Just google Canvas Crayon art and you'll see several examples.
Great Post JenM. The Principal for a day is always very popular. We found that Guidance Counselor for the day and Gym Teacher for the day were very popular as well.