Chemistry classes have been plating pennies for a long time, probably with silver or nickel. my guess is it's just electroplated. sometimes you can tell because you have to suspend the penny in solution, therefore the penny may have one or two clamp marks on it, where whatever you were using to suspend it blocked part of the surface from being in contact with the Ag or Ni solution. I've seen some good ones though where you can't see these marks. Chances are this penny is plated = 99.9999%