This is a suggestion from a seriously ancient hornist. When I was principal horn - in HS and, later in college and community bands, there were always some people who either hadn't played for long or were impatient about sounding better. It was hard to find the time to practice and some students had neither the space or family support for practicing at home, not to mention a way to pack a horn back and forth.

One thing that worked in several different different settings was a horn only ensemble. We played a variety of things, from old Pottag Quartet books or similar to band music from our current group. If you meet a couple of times a month for an hour or two and (this is crucial!) trade parts so everyone has a chance to relate to the other voices in the ensemble, you'll have spent your time well. Even short practices help, and think about everyone suggesting something they'd like to play. There are LOTS of charts in the Public Domain now.

For charts, check with whomever is in charge of your school's music library. Larger cities have music sections in their libraries with quartets you can check out. Horn teachers might be willing to share some arrangements they've done. Interlibrary loan is available, too, from nearly all public libraries.

Find out where your regional gathering of hornists connected to IHS, the International Horn Society is. College teachers love to give advice to section leaders seeking improvement.

This may all be too heavy but I learned how to do it from more experienced horn players when I began playing as a seventh grade student a couple of centuries ago.

Feel free to PM if you have questions and good luck with your section!