Sorry guys, I think that's just a rock. It's not anything at all like one of those pestles or a mano. I am interested in for what reason your ag teacher picked it up and kept it. Might be more to the story, which would help with an ID.
Most of the ones I have at the top has a bit removed for holding. At first I thought it was damage but all the ones I have found and saw they all have a section removed at the top.
I think Charl nailed it, nice find.