I know for a fact that they have several cemetery survey books so I would check that first. Often such books have maps to help you.
You might also post this query to the Bexar County board, just type "Bexar" in the "jump to Forum" box in the upper right. Perhaps a local cemetery buff will volunteer to help.
If none of this helps, there are other options but they are more complicated, such as USGS' "Geographic Names Information System" database but it won't give you any history just GPS coordinates. FYI, Northern Hills Elementary School is located on the Longhorn Map, ID#2003136, DMS Lat. 293400N, DMS Long. 0982338W, elevation 889 ft. in case you decide to check that source. You can do a feature search [cemetery] and might come up with one near those coordinates and at a similar elevation. Unfortunately, old forgotten cemeteries are often not contained in the GNIS database. Finally, local genealogical/historical societies can often help too but I would try the library first before complicating things.