My answer is always yes. No matter where you are in your musical journey, it is always helpful to have a second set of ears to listen to you. For young musicians in middle or high school, lessons can help take your playing to new heights. Band directors only have so much time to dedicate to each student and taking lessons allows you to gain one on one learning experience with a professional that is extremely knowledgeable about your instrument.
I can only speak for myself but lessons are very casual. I try to make a positive environment where students are free to make mistakes and experiment. My lessons are not rigidly structured but are set up in a way that we can have the flexibility to target problems as they arise and learn to mitigate them, while learning fun music at the same time.
During lessons I teach a good mix of solo repertoire and etudes. Etudes are basically small solos that are used to get better at very specific techniques. I also have warm ups that each student does. For advanced students planning to take college auditions, we will also work on orchestral excerpts.
The following is a list of possible books that we might work out of depending on the students skill level:
Pottag-Hovey Methods for French Horn Book 2
Second Book of Practical Studies by Getchell
60 Studies for Horn Book 1 - Kopprasch