I took part last year in the final phase of selection at Waterside for BA's FPP. Let me tell you something which stood out to me. Everybody there had a degree, or was in his/her final year of studies in a University degree. There were even candidates a little older with some employment experience. In addition there were a few employees within BA who were there too as part of the whole selection. There were absolutely no guys/girls who were there straight out of high school.
If an airline has to choose between somebody who's just finished high school and somebody who has a degree plus a lot of qualities and experiences gained from having done a higher education... I think it's pretty obvious a recruiter would pick the latter.
A minimum requirement at some airlines is to have at least a bachelor's degree to be eligible for their cadet program. My advice would be that there's no rush, get a degree first, you'll still be young at the end of it to apply to do flight training! It will look amazing in your CV and will help you stand out from the crowd when applying and it's also a good backup plan!
Good luck!
