Some require high school, some require a Tertiary qualification. It really depends on where you're located.
IMO, there's no requirement for anything other than a high school leaving certificate. It's just not that hard to fly an aeroplane. IMO (again) ATPL theory is equivalent to first year University.
You mentioned that you're wanting to retake some qualifications to get higher grades. If you've already passed, then I personally wouldn't bother. I cannot imagine a situation where your competitiveness is compared to another candidates on whether you got a "B" vs a "C" in Physics.