Depending on the age of the kid, I doubt I'd let them walk home on their own anyway, but lets roll with that.
Call the school, go to the school, search the route, call the police.
Honestly, I'd rather have a kid who is embarressed because I made a big deal of them being late home than have the worst situation happen because I didn't make a big enough deal of it.
Related story- me and my mum were once going to get my brother (at the time he was six) from school and we were told he had already been picked up.
At this point we're a bit freaked because in his school supposedly only those who have signed the collection form can pick up the kid (so, me, mum, stepdad). We asked who had picked him up and we were just told `A man`. Stepdad was at work so it couldn't have been him.
I have never felt so much panic ever.
My mum jumped in the car and told me to head home and wait there for if he comes home and then she goes off searching the area.
Anyway, turns out a family friend of ours had picked my brother up because he was heading to our house anyway, but the school still got an earful for handing my brother off to someone who was not authorised to pick him up.