While managing the Life Lab Garden Classroom I have learned what makes a good sign - a good sign is one that you don't have to repair year after year. Unless of course you want sign making to be a project for your students year after year.
Labeling plants helps kids to learn about them and connect what they eat with what is growing in the garden. It isn't just for the kids either. Adults - parents, teachers, and others often say "wow I never knew Artichokes could get that big" or "Kohlrabi sure is crazy looking". Labeling herbs often encourages folks to smell them, as if they were just checking to make sure that mint smells minty. Having other interpretive signs makes your garden more of an educational site and "teaches" 24/7 to visitors of the garden.
Where you put your sign matters. Signs that face the sun will fade much faster than those turned away from the sun. Is your sign post in an area that is often wet? If so consider rot resistant posts like plastic lumber or metal. Setting wooden posts in concrete can extend the life of a post considerably. And don't forget your audience, make sure to install your sign at kid level so they can read them.
Signs don't have to cost much and most can be made with reused materials. See what I am talking about in the photo gallery of school garden signs from across the state and nation. If you are looking for more professional type signs check out these garden sign designs that can be fabricated on metal or vinyl.
Here are some of my favorite garden signs you can see in the gallery:
One last sign tip to share is Never Tear paper. This "plastic" like paper can be photocopied or printed on and is much more fade resistant that normal paper that has been laminated. If you have digital images that you want made in to signs it is is a good way to go. Staples sells this product and wouldn't you know, there is a site called waterproofpaper.com
Garden Signs is one of many school garden photo galleries.