I know your field is really around the 11+ area
Actually we do everything from Infants up to the 6th form - it's just that most of the questions we get relate to 11+!
You're right that 6th form appeals are a rarity - I suspect that many parents don't realise they have a right of appeal (and that some schools are failing to make that clear).
I have written before about the point you raise:viewtopic.php?f=18&t=29654&p=349564#p349564
Unfortunately, if the issue is about choice of course - rather than entry into the 6th form - it is unlikely to be a matter for an appeal. (I'm afraid choice of course would be considered an internal matter.)
If parents are aggrieved (and I can well understand why they would feel that way) the correct procedure would be for as many of them as possible to make an individual formal complaint to the school. (Start by asking for a copy of the school's complaints procedure.)
They could also consider approaching individual governors for support (especially any parent governors, because they ought to be representing the interests and concerns of parents).