There are several Englishs schools in Puerto Plata.
Alic is one, Johns English School is another one. Another just opened close to where I live on Duarte Street, they have afternoon classes only.
See them I check them out.
Many also take private lessons in English.
To begin a school... depends private or public. If it is private, it is runned as a company, association etc. Then you need licens and the rest of the paperwork, I am sure a lawyer can help you in these matters with what you need exactly.
If you are not in it for the money, I am sure the are public schools in Puerto Plata or volunteer organizations would be interested in having an English section maybe within their school program.
Even though English is taught in schools already.