I don't think you need to hurt or kill the tree by putting a piton in it. Utilize a sling and a carbiner if you want to simulate clipping. Go find a rock with a crack in it if you really want to drive a piton in. Plus, pitons aren't used these days really anyway. Bolts man bolts!

i would trust a sling around a sizeable branch over a piton hammered into wood. if you're just teaching them correct technique for placing pitons, fine. but i wouldn't climb on it. creative problem solving is an important skill to a climber. maybe the best thing would be the toss the rope over the that first branch, adding a sling as the climber progresses. this brings back thoughts of a favorite climb at City of Rocks, Columbian Crack. its a classic hand crack that starts with what seems like an unprotectable chimney behind a boulder, solution: toss the rope over the boulder climbing 15ft to the first protection on TR.