Nowadays, many parents and children alike do not have to worry so much about any pinching going on during the school day because of the fairly new and welcome trend of anti-bullying policies supported by school systems. As a parent of one boy with special needs and another one who is super sensitive, I'm glad that our school system has adopted a no pinching policy. I think that having such a rule in place for March 17 is a good way to avoid a whole lot of trouble. From a child's point of view it might be fun to pinch someone, but it isn't so much fun to be the recipient!
Anyway, in case you are curious, here is a link from Yahoo that gives a likely explanation of how the whole pinching tradition on St. Patrick's Day began. Happy St. Patrick's Day!