If they do not like your idea, they will hate what we have, but it sounds like they have their heads in the sand. Only Authorized Faculty Staff and Students are permitted on school property. Parents and visitors must report to the main office to obtain permission to be on school property. Unauthorized persons may be subject to arrest for criminal trespass. NC General Statute 14-159.
Signage is an important part of school safety and security efforts.
The situation you describe highlights different philosophies about this issue. There are many schools that put up signs that are similar to what you offered. Violators will face federal and state prosecution. The difference in philosophy may stem from an approach which attempts to be inviting rather than punitive. Some schools are using signs that are more positive.