We did have a few cameras that supported SDXC, but the majority didnt. It was easier to just use SDHC and not have to worry about what card works in what camera. Besides, 32 GB was plenty because we almost never shot in RAW (high schoolers aren't professionals); I think only me and 3 others knew about working with RAW.

So to answer your question, space wasn't a premium, and because of the technical and human limitations of SDHC vs SDXC. I'm sure some of those cameras had firmware updates that would add SDXC support, but we didn't need it.