The round going off out of chamber would result in the casing exploding, and the shell probably not going anywhere.. the casing exploding would send shrapnel through his upper body for sure. I ahd this happen with a .22 Long Rifle round in my down vest pocket.. set off by a 9 volt battery. It sent feathers, and shrapnel every which way. LUCKILY the shrapnel took the path of least resistance and went away from my body. If it was an inert grenade round the same thing, except the classroom would be covered with bright orange powder. If it was just the round itself, ie had been fired and found sitting on the ground, it would have killed him, and a few kids in the first couple of rows.. (15m kill radius on the 40mm grenade).
Not sure what the A-10 shoots as far as calibre, but thought it was either 20 or 30 mm.. and most of their rounds are tank killers, non-explosive penetrating rod.
40mm is usually associatted with grenade lunchers, M203 single fire.. or Mk19 Automatic Grenade Launchers.. There are some AA 40mm guns.. BOFORS probably the most famous of the 40mm AA..
Like I said, other then an empty shell, with just a primer, it doesn't sound right..