We can't tell you chances, because we dont enough about you. I have written many posts on this before so if you search im sure youll find them!
If you are applying on the grounds of poor schooling, there is the additional stipulation that no one from your family can have gone to university.
I applied for medical reasons but its still the same. Provided you qualify on both accounts, (both lack of university education in the family and not having many people from your school going to university, ie even if they fail ofsted, if loads of people still go to uni you are not eligible) then by all means go for it. You don't actually apply for it, your referee has to fill out an extra form for you, saying how your education was disadvantaged by your circumstances.
The advice given to me, and that i have passed on to others, was that if you werent sure, then to apply for it anyway, and if they didnt think you qualified they would ignore it and treat you the same as everybody else. The average performance of the school is taken into account anyway, so if you are unable to apply CSAS, they will still have that information to take your grades into comparative account accordingly.
Hope this helps.