Ihave to ask, is the teacher trying to slow the plane down on landing or the top speed?
Ihave been flying my Tower40 trainer with a Magnum 52 four stroke and recently with an Irvine 53!

Ifind the ground clearance to be an issue with 12" props and not that much fun with 11" . Iprefer 10" props to keep it from mowing the lawn or striking the ground on a rough landing. Nothin wrong with 3 bladed props and lately I have been using some 10" wood Zinger wide blade props that can really move the air. A 10X8 Wide is currently on my Irvine 53 and it is working fine.
Does your Tower40 fly tail heavy even though the balance is correct?