This means that a C on a B flat trumpet will be an F on a horn. So C-D-E on the trumpet is F-G-A on the horn which is slightly too high for a french horn beginner and can be quite discouraging in the beginning. Unfortunately, some band teachers give up and only offer the opportunity to play horn to students who already played another instrument before. This isn't necessary in my opinion. I think that with the right method, some coaching and a bit of patience, kids can learn to play the french horn just as well as any other instrument.
When I teach beginners, I make them learn C-D-E (concert F-G-A) on the first week and gradually expand their range up to A in 4-5 weeks and up to C in 8-12 weeks which is enough since most beginner band horn parts don't go higher than C or sometimes D.
Band methods usually have a "just for horns" line for each exercise but that doesn't help when they have to play with the whole band. In my opinion, the best thing to do is to tell the horn students not to worry too much about reaching the notes in the beginning and focus on playing the right rhythm and fingerings. If they work on it on individually with the help of a competent teacher, they should catch up with the rest of the band in a few weeks. Also using a method that is more adapted to the needs of the instrument will help them develop the strength and basic technique to play in the band and avoid frustration and discouragement. You can always download the french horn method here if you are looking for one.