1) Pick really easy show music and drill (20 sets maximum)
2) Schedule a couple of weeks of band camp before school.
3) Use the concept of a "drill day" the last day of camp where you learn nothing but drill with click tracks. These are easy to make in garage band or any program like it. I usually learn at least 1/3 of my show on drill day. We usually spend around 8 hours of nothing but drill.
4) During band camp we do nothing but music and marching fundamentals.
5) During your spring concert prep, work up the music for your show after UIL Concert Contest. This makes it to where the students are ready to learn drill in the fall and are excited about coming back in the fall because "they already know the show."
6) Find a compromise on scheduling. If you can't get a 2-3 hour block of time one of the days of the week work on seeing if the students can get there 10 minutes early every day or even more. Also, if people are unwilling to allow band to "have their day" for night practice then just try to schedule a couple of clinics during the season on a Monday or Tuesday and let the coaches know that you have a mandatory event that the band students have to be there a few nights during the fall. I'd gradually grow those "clinics" until it's just about every week.
7) I work in a 2A program that has an outstanding history of both concert and marching band and they never had outside of class rehearsals. This band has won many appearances to state marching contest and a couple of honor bands. Some parts of the state are more competitive and accomplishing these goals with that kind of rehearsal structure would most likely not happen. If your goal is to get a 1 in region, then it would probably work fine to not have outside of class rehearsals in this classification as long as you are efficient and practical in your demands.
Best of luck, there is no "one way" to teach and run a band program. I hope this helps you.