Cant understand why anyone gives a brass monkey's about their school grades.
Flying schools don't care what school grades you got, they just want to see the color of your money.
Get a frozen ATPL, go instructing for a couple of years until you have 1000 TT and no airline is going to care what school grades you got either.
I scraped through school with mediocre grades in a load of crap non-maths and non-science subjects like music and history (back in New Zealand) and then went to do an arts degree which I totally screwed up and failed so badly that the university banned me from re-applying to finish the course.
I've been flying for 12 years now and done a few big interviews including Easyjet, Air Mauritius, several UK turboprop and bizjet operators, and also Cathay Pacific which is supposed to be the hardest interview in the world, and no one's ever even ASKED about my school grades let alone given me any grief about them.
The way it works in this business is that jobs are dished out first of all to all the management pilots' kids, and their kids' mates. After that it's according to whose dad has been in the Freemasons / Squadron leader in the RAF / drinking buddy in the Conservative Club bar for the longest. After that, they go to the guys who've been hanging around the local flying club the longest and kissing the ass of the local chief pilot when he pops in to take his warbird for a Friday afernoon jolly. Once they've exhausted those lists of potential candidates, they start calling people from the CV pile in utterly random order. If they're really organized, they've told the kid that works in ops to arrange the CV pile into some kind of order of who's got the most hours and relevant flying experience - and then of course that kid has put his own CV and all his mates' ones on the top of the pile anyway
Honestly man get some life experience and get some hours on light aircraft, and no airline is going to give a single solitary rat's @rse about what you did at school, I guarantee it.
*Footnote to the above - if you have a pattern of multiple screw-ups and failures of your ATPL subjects and your CPL and IR flight tests, then yes you CAN expect to be given a hard time at interview. So get over the school thing, and work like you've never worked before when you start doing relevant stuff like actually learning to fly.