I teach at a gemological institute. All our students come from varied educational backgrounds. Some are fresh out of high school, but a good number are degree holders and professionals.
Is there any course of study that may give a student a distinct advantage over another? Of course. But because the field of gemology is so wide, and because it straddles BOTH art and science - I think, to succeed, all a student really needs to show is passion for gemstones.
So take whatever classes interest you in high school, because GeoChris is right - you don't need to specialise in Geology to study Gemology. It's very simple - all you need to do to become a gemologist, is to sit for a gemological diploma.
There are several respectable institutes that offer this.
For example: Graduate Gemologist (GG) from Gemological Institute of America (GIA), Accredited Gemologist (AG) from the Asian Institute of Gemological Studies, based in Bangkok.
With your GG or AG diploma, you may opt to sit for a higher diploma - the FGA - offered by the most respected gemological association in the world - Gemmological Association of Great Britain.
After you've gone through these courses, you will have earned the right to write "Gemologist" under your name!
For the record, my degree is in the Arts & Social Sciences, and I hold a postgraduate in Human Resource Management. As you can see - neither of which is at all related to gemology!
Hope that helps! Good luck on your path to becoming a gemologist!
I LOVE gemstones and am a gemologist.