The Four Seasons mosaic by Marc Chagall in Chicago was originally installed outdoors in the 1970s but has since had a glass canopy installed over the top to protect it from the elements.
Part of the reason for the canopy is Chicago’s harsh freezing weather, which is hard on all mosaics, but another reason for the canopy is that Chagall painted additional details on top of the tile in places where his artist’s eye saw that that something more was needed.
Everything (except being boring) might be legal in visual art, but in mosaic, not so much. When you are making something to withstand the elements or to function as an architectural surface, you really have no choice but to use best practices and standard methods and materials. Otherwise, the artwork won’t last.
I recommend that schools wanting to make a mosaic mural for their school consider “crazy quilt” displays that are assembled from individual mosaic projects all on the same size backer, say anywhere from 6×6 inches or 12×12 inches.
This allows each student a real art experience (making their own design) instead of just being a worker bee on a group project, which runs the risk of teaching mostly the craft/shop aspect of the process while being too light on individual expression/design.
A good compromise is to have the class work on a group project to “learn by doing” under supervision and them have them do small individual projects afterward. These individual mosaics are then arranged as a “crazy quilt” frame around the central group project.
It is important that any school’s visual arts class or art project actually be about students doing art (individual design and expression). Don’t lose sight of that in your school’s project.
When the students start gluing tile, you will be pre-occupied with showing inexperienced people how to glue without making a mess and won’t have much bandwidth for making sure everyone is working consistently in terms of spacing.
Showing the importance of small consistent grout gaps and how to arrange tile can be done before glue is ever involved.