At this time, it is NOT MANDATORY to have immunizations to travel in the Dominican Republic.  See "What about immunizations?" on the Senior Trip FAQs.

S.C.O.R.E. states that it is always their policy to recommend that travelers contact their local physician and health department and discuss travel plans when traveling to any foreign country.

S.C.O.R.E. recommends plenty of mosquito repellant with at least 50% DEET.  Permethrin is also effective and non-toxic to humans, used along with DEET.  These products can be purchased from most sporting goods shops.