Super late answer but I think I found it: "Duffy's Jacket" by Bruce Coville. I had to dig it up from an old school book of mine. I came by this question because I was searching for this story as well.
It can be found in Spooky Stories for a Dark and Stormy Night:
The figure stepped into the room. Its head nearly touched the ceiling.
Marie squeezed against my side tighter than a tick in a dog's ear.
The huge creature sniffed the air. It turned in our direction. Its eyes seemed to glow. Moonlight glittered on its fangs.
Slowly the Sentinel raised its arm. I could see Duffy's jacket dangling from its fingertips.
And then it spoke.
"You forgot your jacket, stupid."
It threw the jacket at Duggy, turned around, and stomped down the starir.
Which is why, I suppose, no one has had to remind Duffy to remember his jacket, or his glasses, or his math book, for at least a year now.
