Some playwrights add a lot of stage directions (Neil Simon was famous for this); some only minimal ones. One reason is that directors and actors often choose to ignore stage directions, which might include specific actions, gestures or emotive suggestions, which limits the ability of the director or actors to experiment.
In The History Boys, much of the play works because of the relationships of the students to one another, which can be conveyed via facial expressions, shared glances, gestures and physical proximity. Sexual tension is an important component of the play, both the sexual preoccupations of adolescent boys, and the issues of homosexuality underlying the action. Because of this, the ways in which characters do or don't touch one another, their vocal tonality, may go a long way towards communicating information not clearly spelled out in the dialogue.