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

I'm 16, and I have C above the stave and C two octaves below middle C. I've been playing since I was 8 though!

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.

When I was in high school, my range was low F below the staff to G above the staff ( on a double horn). My range has increased though now that I'm in college, I can hit a A above the staff and a low a C below middle C. This is on a single horn though, I haven't gotten to play on a double horn yet since my 2 year break after high school so I'm not quite sure of my full range on a double. On a mellophone though, I can hit a high C above the staff.

For most literature (and I mean 99%), the range described by John should be enough (C 2nd space bass clef to c above the treble clef).

I'm 18 now, and I've been having private lessons at a local music school here in Germany from a professional since I was 8. My usable range (which I can play without to much fear of not hitting the notes) is from F just below the bass clef up to high c. On good days I can hit a d or eb above treble clef (I've hit an f occasionally) and play down to C below bass clef (all on full double).
By the way: all the videos on Youtube where horn players supposedly hit lower notes than that are bullshit. This C is the lowest note on natural F-Horn that is physically possible, i.e. the first harmonic (with all three valves one could get down to Fb, but that's it).

A semi-pro or enthusiastic amateur should be able to play 4 octaves, while being able to hit high d's (on the descant) for very high parts in baroque music.

The lowest note in horn literature is in Josef Suttner's Concert for Horn, 2nd movement, where one has to play a b below bass clef. The highest note is most certainly in Neruda's Concert for Horn that is played by trumpets most of the time. It goes up to g above high c.