It might be appropriate to build it off the standard of Divination spells.
One inch of air or water, or 3 feet of organic material such as wood (or carpet) between the character and the soil/stone surface of the world will block the school from working.
Three feet of organic material is quite a lot, so carpets, wooden floors, grassy fields, and boots won't cause a problem. (Although fighting on a mound of corpses might.)
Metal and stone should count as part of the ground, as long as there are no layers of air or water in between the character and the world's surface
One inch of air or water, or 3 feet of organic material such as wood (or carpet) between the character and the soil/stone surface of the world will block the school from working.
Three feet of organic material is quite a lot, so carpets, wooden floors, grassy fields, and boots won't cause a problem. (Although fighting on a mound of corpses might.)
Metal and stone should count as part of the ground, as long as there are no layers of air or water in between the character and the world's surface