The only English speaking board school in the DR is located in Jarabacoa, La Vega province, about an hour and a half north from Santo Domingo. The Escuela Caribe, nevertheless, has a reputation as a school for children with problems.
There are quite a few English speaking schools in the Dominican Republic, some with US certification and others with out. As far as I know the only one that is a boarding school is Escuela Caribe but it is designed as a reform school where families from the US send their truobled teens. It is a Christian program which tries to help these students within a sheltered environment.