No dad is going to hide under two 18 year old couple's bed. He does not wish to be under there before, during, or after they do the horizontal tango, and even worse he does not want to be found HIDING UNDER HIS CHILD'S BED AFTER THEY DO SAID HORIZONTAL TANGO! How anyone could ever believe that story is beyond me.
Also, while it may well have happened, the photographing of some girl's arse in PE is at best weak banter, and at worst creepy and bullying behaviour. It raises hackles.