The Cumberland (R.I.) School Department yesterday reaffirmed its decision to deny a public hearing to Paul Guilbert, 17, an avowed homosexual who wants to bring a male escort to his high school junior

prom. Supt. Robert G. Condon said the overriding factor was the fact that Guilbert's parents objected to such a hearing. Guilbert's father, Eugene Guilbert, has threatened to go to court if the American Civil Liberties Union tries to represent the youth. The proposed "date" for the prom May 4 is Edward Miskevich, 22, a 22, a senior at senior at Brown University.