Classification by range

Given

studentNumbers<-10;

subjEnglish<-sample(-1:100,studentNumbers,replace=TRUE);

when subEnglish <=0, 'U'
                        <=39, 'F'
                        <=49 'D'
                        <=59, 'C'
                        <=69, 'B'
                        <=79,'A'
                        <=100 'A+'
I can solve this using a series of 'if' 'else' statements. I am  
looking for a simple expression that will give the letter  
classification for every element of subEnglish.