Your main tuning note reference should be F# with the octave key. The embochure should be a cirle, not an oval. Try having the student say OHH over exaggerating the sound to form the face correctly. Place the teeth on top of the mouthpiece fold the bottom lip over the bottom teeth just a little so there is still red showing, and close the sides in like a drawstring. The OHH sound should also help them place their tongue at the correct shape. If the student is not using enough air or lip pressure the sound will be flat. Too much pressure and the sound becoems sharp.