I'm an elementary school teacher and I have a few parents who do that (since we have the same policy) -- Mom A volunteers on Monday mornings while Mom B watches the kids...then Mom B volunteers on Wednesday mornings while Mom A watches the kids! The only "problem" is that if Mom A has a sick kid or an appointment, it takes Mom B out of the volunteering mix for that day, but it's definitely better than not having volunteers!
As far as the school goes, it sounds like you've already got a great dialogue going with your son's teacher. Yes, kindergarten is definitely full of plenty of procedures and rules...but once the teachers have kids practice with the procedures with books and materials that ALL kids are able to understand, it's easier for them to differentiate with the levels of the materials. Hopefully it will be the same way in your son's classroom!
