I'm teaching trumpet, french horn, and trombone this year in a school setting. The students are around 11 years old and have all been playing for about a year. They can read music with some success and have a decent, characteristic tone. But they are seemingly unable to rise above a very narrow range. They are able to match pitch with me when I play along when I play [written] C, D, and E [sounding F, G, and A below middle C].
I've tried having them buzz on the mouthpiece without the horn, focused on good posture and air support, and demonstrated the sound their mouthpiece should make but we're not really getting anywhere. They are musically-aware enough to know that they're not playing the right notes and I don't want them to get frustrated enough to want to quit the program.
What are additional things I can try to support young brass players (or horn players in particular) in gaining that first octave of range?