Help with CSA

I've agreed to help my brother-in-law to find some informationand hopefully reduce his CSA payments.
His two boys are 14 and recently turned 17 and both in education, the youngest full-time.
The ex-wife has been difficult since the split 9 years ago, fighting all the way.
The eldset stayed on in school to re-sit his exams and attempt to progress with his education. He was advised to go to college and study a trade or take an apprenticeship, but like lots of kids these days, he is unsure about what he wants to do.
His mother is very domineering and has insisted that he stays on in school, he is afraid to go against her. We are convinced that she is influencing him in order for the CSA and Child Support payments to continue.
My brother-in-law has always exceeded his access to the kids, and on reflection probably has them staying with him nearly every day off he has, this includesmost of his days off, holidays and even when he is working (he works 12hr shifts). It probably amounts for 20-30%.
Although he has attempted to tell the CSA how often he has the children, in an attempt to reduce his payments, the CSA ask the ex if it is true, and of course, she denies it. He has even kept a diary and bus tickets, but they tell him there's nothing he can do, and that he would have to pay to fight the decision.
It has recently come to a head as he is struggling to maintain his mortgage payments.
We have also recently discovered that the eldest should be in full time education in order for the payments to continue. By definition on ths CSA and Child Support websites, this means attending at least 12 hrs per week of supervised lessons. Can this please be confirmed.
We have also realised that he doesn't acheive this figure, in fact, he is only doing 3hrs per week and that this was also the case during his last year at school.
Can anyone please advise if you think we have a case appeal and reduce his CSA payments.

is he supposed to be attending and just skipping classes?

No - He only has 3 hrs a week scheduled through his school lessons

and what sort of education is it? a college course?

It's two subjects in school.
Business Studies & a compulsary Welsh Assembly course.
Wk 1 Wed 1 lesson 1hr
Thur 2 lessons 2hrs
wk 2 Tue 2 lessons Business Studies
Friday 1 lesson Business Studies

Average 3hrs per week

i assume that you have phoned the csa and told them this and there is nothing that they can do, a child is still deemed as a child until they either leave education and start full time employment or child benefit ceases to be in payment. if neither party can agree then the csa will use the child benefit as 'best evidence' to make their decision which i assume in this case will still be in payment as the child is still at school. you will only have appeal rights one calender month after an assesment has been completed if this is not the case it is highly unlikely that an appeal will be heard. if you have spoken with more than one person at the csa and they have told you that the decision is correct then it most likely is.

he has only dealt with the CSA regarding the amount of time that the kids spend with him. The latest news regarding how many hours are spent in education have only just come to light after I did some digging regarding the definition of "Full time Education" and looking through many websites.
This part has yet to be presented to the CSA. I am guessing, based on the information that I have taken from the CSA site and various other sites, that he fails to meet the criteria, and that his mother has also failed to inform the Child support agency of these changes since he finished his compulory education in the sumer of 2008.