Here is the deal...
The following cities in the Dominican Republic may either have a school that teaches spanish or are nearby one.
PUERTO PLATA-SOSUA-CABARETE AREA
This is one of the playground of the world. Tourism is everywhere with everything that that applies. The area does have some local and international flavor. Puerto Plata is a decent size city, it has a nice "downtown" or old part (though few buildings are really old in this town). The area from Puerto Plata to Sosua to Cabarete offers most of the necessities one might need to make this their permanent or temporary home with out giving up too much luxuries. This trio of towns have a decent nightlife, but of the three probably Cabarete is more gueared to the young croud. Cabarete is a one road town (meaning the entire town is on one road, literrally). It's the windsurfing capital of the Western Hemisphere, so it has a hip and young ambience. Lots of young folks from Europe and the America's hang out here and can make life fun and interesting for anyone willing to live here. The beaches are excellent, in most cases better than Florida's due to the mountain back drop and "half moon" curve most beaches tend to have. The locals are for the most part friendly and there is lots of things to do when you are not studying. However, keep in mind (like everywhere in the world) if you don't go where the action is you will be bored to death.
SANTIAGO
It's a city with a population of about 800,000 possibly more. It's a prosperous town with alot of American influences in it's development. For example, many of the shopping centers along Avenida Juan Pablo Duarte are in the American style of strip malls. The nightlife here is good, but not better than Puerto Plata area, however, Puerto Plata is only about an hour or two away. There are extremely few tourist in town, that may be good or bad, depending on your preference. There are some nice neighborhoods in town, nice stores, a couple of new shopping malls, Mc.Donald's, Burger King's, Pizza Huts, etc. The town is near a very appealing but small international airport. It's very close to the Central Mountain range which is the adventure sports capital of the island. In the mountains you can go mountainbike riding, horseback riding, water rafting, tubing, canyoning, hiking (like the tallest mountain in the Caribbean at over 10,000 ft). The city is only a 2.5 hour drive along a smooth four lane super highway from Santo Domingo. It's a short drive to the Samana peninsula where the word paradise fits exactly into the atmosphere there with palm clad hills, secluded idillic beaches, and friendly people. It's a nice town with nice things to do, should really consider it, but don't make it your final decision until you've visited either Santiago or any other town you have in mind.
SANTO DOMINGO
This is with out a doubt the most exciting city in the entire Caribbean. Over 3 million people call it home. It has an endless number of bars, discos, nightclubs, restaurants, casinos. It also offers bowling, horseraces, greyhound races, cockfighting, car racing, high quality baseball, tennis. It has it's fair share of beautiful and expansive parks with jogging trails and the like. Lots of nice areas, but like all major cities in the world, there are some pretty depression sections also. Lots of people from all over the world (mostly dominicans though) and from all walks of life. Rich folks on spiffed up SUVs are as much of a common sight as are "chiriperos" or push cart vendors scratching a living by selling fruits and lottery tickets. There is alot of potential in this town for people to make, just get aquainted with the right folks. There are many golf course within striking distance, it has a beautiful palm lined avenue skirting the Caribbean sea. Lots of Museums, an aquarium, a nice zoo, monuments ranging from obelisks to a huge pyramid-cross structure named Faro-a-Colon, commemorating the discoverer Cristopher Columbus. It has a Colonial Zone where many buildings were built in the 1500 and 1600. You can see where Columbus hanged around and bragged about when he used to live. His palace is still standing, many palaces, nice squares, very nice area with open air cafes and interesting shops. There are about 8 to 10 shopping malls ranging from the chaotic and very unoganized as Mercado Modelo to the super exclusive marble everything glitzy glamour of Plaza Central. Absolutely anything you could ever want is available somewhere in the city. The crime level is relatively low for a city of it's size and economic malaise, but stuff do happens, most people only hear about stuff happening thought, they don't really witness it or are part of it. Just take care as you probably do in Spain or back in Florida and you'll be find. Many people say Santo Domingo has a Miami-Rio-Spain thing going with an African tint. Get aquaited with merengue music because it's everywhere. When I say everywhere, I mean everywhere. The cost of living in Santo Domingo could range from the cheapest you'll ever live on to the most luxurious and pocket sucking lifestyle ever, just depends what you want in terms of luxuries and comfort. Overall it's a pleasant city.
Now you have an idea of what these three places have to offer. All you have to do now is visit the places on your own and make a pick based on the information you got and on how comfortable you feel. Enjoy the Dominican Republic to the fullest when ever you decide to follow Columbus footsteps (from Spain to the new world).