Trainer are set up with typically 6 degree positive incidence in the wing.

When they are flown slow the fuselage and trim is fairly level but when you fly faster this all changes

Trim the plane as directed in the early post

If flying along under high power and the plane immediatly tries to climb, you have too much positive wing incidence that is compensated for with down thrust.

Ideally the plane's trim could be set up to remain the same under power or in glide but they seldom are.

The Telemaster was a trainer with a lifting stabilizer to keep the trim the same at different speeds. There was also a Lazy Ace Bipe that had a lifting stabilizer for this purpose.

I put a shim under the rear of my LT-40 wing to decrease the positive wing incidence and settle the climbing tendency some.