I'll put my money on you not tightening the prop nut enough. One nut on a two stroke is enough, you just need to sock it down REAL tight.
Not sure what you mean by "I am a flight instructor" though. If all you've flown is an Aerobird, and this is your first glow plane, you really don't have nearly enough experience to be an instructor. (Unless you're talking full scale.

)
Welcome to RCU!
Dennis-