One thing that I would suggest is to look through some old year books of the school that you are attending (as not to use one that has been previously used.) Another suggestion would be to recruite some creative thinkers to submitt ideas. Most of the mottos that I have seen lacked creativity and most of them rhymed. Ryhme is not essential. Our High School senior class motto?
"We're the greatest class alive, Senior Class of 75." So what was so great about our class? Nothing really. Our Freshman class was said to be the largest (125 students) to enroll at our high school at that time. Four years later there were 89 graduating students. (11 attended college and 5 enrolled in the military.) The class president became a lawyer and later set up his law office in the same home town. One of the guys became a Veternarian in Nashville. One graduate became a CPA and eventually a CFO in a major corporation. One graduate became an engineer. Four graduates became school teachers. Two became nurses. My best friend graduated from Murray State Kentucky and became a farmer. All in all we were just an ordinary class with a bad motto.