Just a couple of questions:
1. Do they get lessons or is it just a group band
2. Do they get a grade for the class?
3. Why sign up each semester?
Here is what I am extrapolating from what you have read. The kids may or may not get lessons, but your band is more of a club than a class. If they get a grade for the activity and their parents rent the instruments that should give you more leverage to keep kids the entire year.
How often do they get to “show off” what they have learned? Let your concerts be your recruiting tool, make what they play sound good and people will start flocking to the activity. Also try to get the kids talking about band. The kids will listen to their peers before they listen to us. Show off your talents to the school to show them what they can accomplish. Once a month I play at one of my elementary schools a different instrument and use SmartMusic to accompany me. The kids love it and it exposes them to what is possible if they work hard.
Hope this helps.