Okay, so maybe it wasn't a student... The woods teacher was using at the time!
Obviously the poor hammer has been through the wringer. Looks like it had started cracking a long time ago- Thinking that this must be the weak spot and it has fatigued over years and years of constant (miss)use.
It's been sitting on my desk for a day or two- Every. Single. Student. has to ask me how I broke the hammer.
"I got a little upset with (insert students name here)..."