I’m speechless that anyone instructing in a high wing Cessna (can’t speak for other types) suggests that extending flaps extends a glide. It isn’t what the POH says and I’m relieved to read above that everyone takes the approach that I had drummed into me-only extend flap when you know you’re going to get to the field
Once flap is extended it should never be retracted until you have a positive rate of climb. Very nearly learned that the hard way.