Kid traces his hand on his desk. Kid traces his hand again, this time with his middle finger extended. Kid is huge. So is the traced hand. He tries to blame a much smaller kid. Teacher places kid's hand in the outline and it fits like a glove. Kid laughingly admits he did it, then two minutes later tries again to blame it on another kid.
Apparently, not only was Teacher born yesterday, but with mental deficiencies in the reasoning department.