I did my PPL during sixth form, and I wouldn't recommend it. If I could go back it's one of the few things I would change. Ultimately neither got my full attention, so both suffered (although my A-level results suffered the most).
Ultimately it didn't matter since I now have an airline job, and A-levels seem pretty unimportant now, but I did have a job before I started my CPL training where A-levels were definitely a deciding factor. So I guess my advice is don't spread yourself too thin, there's plenty of time after school to sort out a PPL.