(5)The time during which a youthful offender or child is absent from a reformatory or industrial school in pursuance of a license under this section shall be deemed to be part of the time of his detention in the school:Provided that, when a youthful offender or child has failed to return to the school on the license being revoked or forfeited, the time which elapses after his failure so to return shall be excluded in computing the time during which he is to be detained in the school.
