He probably wasn't sneaking, skulking, prowling, stalking, or anything of the sort 
Heck, would anyone here honestly think that calling 999 was the correct response to seeing a DPD driver get out his van and proceed to enter the school via the incorrect entrance? Seems a bit over the top to me! Surely you'd just approach and say, "You need to go to reception... I'll take you there," or something.

Heck, would anyone here honestly think that calling 999 was the correct response to seeing a DPD driver get out his van and proceed to enter the school via the incorrect entrance? Seems a bit over the top to me! Surely you'd just approach and say, "You need to go to reception... I'll take you there," or something.