There are often diplomat-oriented international options if you look for them. I did the IB diploma program, and one of my school friends went to the American School in London. There's probably a way to get some equivalent where you are. I can't do the research for your specific situation, but a quick online search yielded a GED test center in Santo Domingo along with some very affordable GED prep programs.
Your embassy/consulate should have someone you can contact who can point you in the right direction if you can't find anything or need external help. I've worked with state department folks professionally and the ones I've worked with have all been great guys with a lot more knowledge than you'd expect!
If your folks are uncooperative, it might take some extra creativity and determination, but every obstacle is surmountable with some perseverance.