If you can identify the manufacturer, go to the library and look them up and contact them. Perhaps contact several major ring makers. They all have lists of all the high schools and may give you a few schools to check out. Ask some of the local high school's office staffs for a few ring manufacturer name and numbers they get quotes from each year. Start contacting them.

That's great you are attempting to return the ring. It may take a bit of sluth work but will be well worth it when you succeed. - Ed