I was openly banned from entering sixth form due to poor GCSEs (3xBs, 4xDs, 2xEs), even the local college wouldn't let me to A-levels, so I got stuck with a vocational qualification instead. It didn't stop me. Still managed to get into Uni and spent years working toward my goal. Now happily sitting in the RHS.
I know a training Captain that dropped out of school at 16 with virtually no qualifications at all (wouldn't recommend going that far).
Passion, Patience and deep pockets are all you need
Good luck