Why are you "so dead" if your MIL finds out your child's school mascot? Will she argue with you to change the child's school JUST over a mascot?
If so, tell her that you and your husband are focused on your child's EDUCATION and what happens in the classroom, and not on something as trivial and meaningless as a cartoon dragon.
If she brings this up again, say every time with a big smile, "Sorry, we already discussed this issue, remember? What matters is the good education that our son gets. That is all that matters, and we're not up for discussing the mascot issue again, not now and not in the future." And stick to that. Walk away from her if you have to and say that the subject is done. If she is so obsessed with this issue she truly needs more to do in her life. If she makes your life miserable over it, it is your husband's job as her son --not your job -- to take her aside and tell her this topic is permanently closed because you as parents, not she, decide on your schools. (And would you actually have canceled your husband's university enrollment because his parents don't like a mascot? He is an adult!)
By the way: One of the country's greatest universities, Duke University, has as its mascot the Duke Blue Devils. Duke is also the home of the Duke Divinity School which has trained many generations of Christian ministers. I have never heard of any of those ministers and pastors yelling that Duke must change its mascot. And Wake Forest University is a strongly Baptist Christian school which also trains ministers, but its mascot is "the Demon Deacons" (a deacon is a kind of minister). Again -- a "devilish" mascot for a CHRISTIAN school. In other words, Christians can have a sense of humor and not get all in a twist about the mere words "devil" or "demon" as if just saying them will conjure up the real deal. If she gets into it, refer her to Duke and Wake Forest and the many ministers coming out of those schools! But seriously - I would not even address this with her and would change the subject instantly EVERY time she brings it up.