For most all of the music for horn I've seen in highschool the range is usually from G below the staff to G right above (not all in one part/piece, mind you). It's probably a good idea to be able to play a few notes out of the range you perform in so you can get the extremities more easily.
The absolute lowest I've ever seen was a G two octaves below the staff, but that wasn't in any music for my highschool band. And the highest I've ever had to play was a B-flat above the staff in a solo by Strauss.
The Horn's standard orchestral range is from the C above the staff to about 3 octaves lower but music by some composers go way above/below that as well. The Schumman Kozertstuck goes to the E above the high C in treble clef, while the Mahler Symphonies go right down to the E below the bass clef.
For standard band pieces, you probably won't see anything above high C or below the F below the treble staff. Ferrer Ferran loves giving the horns high B-flats and Bs, and I've played a high C-sharp in a piece by Bert Appermont. The lowest I've personally seen for a band piece (not orchestral transcription) is probably a low G, as Frederick says, but being able to reach the low F is safer.
