You will have to go to the lake in the mornings and meet Jimmy O' Neil.
Tell him where he found the old headmaster's body . After a few days Jimmy will come to your office to investigate the place where he found the body .You and Jimmy found nothing but then Jimmy's dog found the wallet of the old HM. You will see a four digit codes on the paper in the wallet for the safe in your office. You will know what to do next.