So you just paid the money and left them to it? I bet the school can't quite believe your lack of involvement and lack of committment to your DC's education.
Don't have personal experience but surely teachers at independent schools are used to fee-paying parents being very involved in their DC's education, having strong teacher-parent relationships and parents putting in the effort to ensure that a sufficient study ethic is nutured, maintained and put into practice at home.
If it's a case of your DTS's not being very bright then that's another matter. If this is an issue with all their class mates and there is a real failing with the school I apologise.
Otherwise, instead of withholding fees teach your DTS's about the consequences of their actions, and the importance in life of taking responsibility for one's self.
Admit to the boys that you let things slide a bit on your part and make them own up to the fact that they have too.
Make them see that only they can turn this around and it will be a real triumph if they do. I'm sure it is do-able (I missed shed loads of school in Y10 due to illness, was predicted a D worked my socks off and got an A).
I hope they do well. Good luck.
You aren't doing them any favours at all by letting them think that this is the school's fault. And sorry to commit the crime of being judgey, but I suspect they've go the ill-conceived idea that if- you're-paying-for-it-you-don't-really-have-to-work- at-it-attitude from you.