Place mouthpiece on closed lips, 2/3 bottom lip, 1/3 top lip.
Keep lips in, not relaxed at the sides of the mouthpiece. Double check "corners."
Pitch buzz goes higher by changing the ANGLE of the lips - curl the bottom lip "in" slightly to play higher.
Simplify your instructions if you're having trouble. This approach has worked for me 99.9% of the time with beginners. It's also something that can be taught well in about 10 minutes flat.