There are many private schools in different parts of the country that do a reasonably good job. Obviously there are ones that don't. San Pedro de Macoris is a large enough city that there must be several quality schools.
http://www.laspalmas.edu.do/, Colegio los pininos, and Centro de estimulacion y desarrollo Psicopedgogico ( CEDEPSI) as well as the Adventist school. Have you checked all these private schools in San Pedro. I think there are around 200 private schools there. Ask a few of the dentists, doctors...etc where they send their children and why. You might be surprised at what is available.
you might not like the idea of sending the children to a religious school but I sent my children to Catholic schools in Canada and all three of them are unreligious, but did appreciate learning that aspect of living.