How's the scheduling at your school? Will it be any different? In my district, 6th grade is at the middle school, band is just one of their classes on the bell schedule. 5th grade is at the elementary. If we started there, it would either be a pull-out or just part of their specials rotation with computer/library/whatever. I taught in a district where 6th grade was on that schedule, and I felt like our 6th grade beginners only made 1/4th the progress of a class that meets every day year-round.
What's the sports situation? For us, 6th grade is when sports really pick up. We fight to keep kids in band and get them to come to the few after-school things we have. Maybe if you could hook them in 5th before they have real school teams, it would help.