74 Where it is impossible to have schools of singers or where there are not enough choir boys, it is allowed that "a group of men and women or girls, located in a place outside the sanctuary set apart for the exclusive use of this group, can sing the liturgical texts at Solemn Mass, as long as the men are completely separated from the women and girls and everything unbecoming is avoided. The Ordinary is bound in conscience in this matter."
Source: Musicae Sacrae (Vatican.va)