Is this in terms of composing/arranging something for the horn? If so, I'd be careful about giving a common-or-garden high school horn player too much above a (written) D or E (ninth or tenth above middle C).
(Disclaimer: I'm not a horn player but I've written for the instrument on occasion.)
I am a horn player.
Depending on the school districts music program here is what you want to know.
And be aware this is the high end and you can't leave them up here. Just a few notes in the piece.
A poor district without a good feeder program your horn player should be able to play their written 'e' top space of the treble clef.
A good music program with people who have played horn a few years and take private lessons should be able to play their written 'a' just above the treble clef.
A really good music program will have Horn players who can play their written 'c' above the staff. (And I mean really really good program, where private lessons are a must).
You are talking high harmonics here. Anytime you write above 'd'(their second line from the top of the treble clef) you are risking it not being played by a high school player and you are getting them very tired much faster.
Also take note their written G thru Bb just below the treble clef is a very difficult set of notes to play melodies on, especially if sluring. Above the Bb is okay and below the G is easier, but that little grouping is difficult to sound good for younger players.
umm, i think a good range for a high school student, will be middle c to uper, the one above the staff, a por que it will be easier if you have to play romeo and juliet or canyon of the gods for marching season, soo i think that is a good range
The highest note I've ever seen was an A above the staff, but that was only very occasionally. The lowest note was a G below the staff. So I think you'd be safe with a consistent range from G below the staff to G above it.
Two sets of rules.
There should be 2 sets of rules/expectations here. One for the composers and one for the Horn players.
Players::
I like to see my high school level horn players go up to their written C above the treble clef. And I like to see them play down to the low C, second space in Bass clef. This is a good high school player who is interested in going on to college.
Hobbyists should have the A above the treble clef and the low C second space bass clef.
COMPOSERS::
If you are writing for high school level players you need to decide what level you are writing for? Grade 2 vs grade 4 will be different. You need to decide waht medium you want to write in. If you want to be known for challenging grade 6 music then write high C's and double low C's below the bass clef. But your music won't be played much. If you want to be played a lot go for a grade 3 level. Keep most of your stuff on the treble clef and go up to G above the staff rarely. Don't go into bass clef but feel comfortable going down to the F three ledger lines below the treble clef.
You can make other exceptions if perhaps you are writing a specific piece for a specific group. Then talk to the players and see what they can do.