I pretty much totally screwed up at school. Was doing Biology, Maths and Chemistry. Dropped biology as hated it even more than the other two and left school with a D in Chemistry and an E in Maths, went to Uni, amazingly I got into one. Did that for a few weeks then dropped out. Could not get a descent job and spent the next two years working in factories trying to scrape the cash together to fund some training, living with parents. On minimum wage this was going to take a long time. Nothing worse than working in a factory for 12 hours from 6 till 6 overnight. For those two years I thought **** pretty much every day, if only I had done better at school, gone to Uni, joined the RAF (got a flying scholarship, passed the aptitude near the top of the applicants and was told to come back after uni, join the UAS and I should be ok) but as had only a D and E at the time had not really proven myself. I am not stupid, well not mentally inadequate, just didnt care when I was at school so didnt put in the effort.
Eventually my dad decided to help me out and I ended up two years after dropping out of Uni in the states doing my training. If he had not done that I would still be in some ****ty job in the UK and by ****ty I mean ****ty, one I had was shoveling **** out of a filtration tank at a waste treatment plant during the filter refurbishment :-( I am lucky that he stepped in.
So after I got my FAA ME/CPL/IR I managed to find a job in a 206 and 7 years later after working in some of the worst places in the world, Kabul, North Africa etc I had managed to amass almost 3000 hours mostly in twin turboprops. I am now holding sandwiches and babysitting the Airbuses for Cathay as a second officer, not complaining, the pay and housing are pretty good and it is a good company compared to most.
So as you can see it is possible (was possible) without good academic results. If I could do it all again I would try
hard at school as could be so much further in my career than I am now. Looking back I have enjoyed all the flying I have done and all the places I have seen and lived but without a doubt I would have rather got good grades and found a job in the right seat of an A320 or 737 10 years ago as now would have been a Captain with them or FO with the likes of VS or BA. You cannot find a good job with low hours and having done **** at school, that is a fact, I know. You can find a job (could) and then get enough experience working your way up through single pistons, twins, turboprops to make it to a descent company but trust me it is hard. Eventually companies don't care about how you did at school as it really is not important if you have proven yourself in other ways, didn't even come up, the D and E at A-levels during my CX interview. It just takes a lot of time and effort and luck to make it not be a problem anymore. The decision you are making now will affect you for the next 5-10 years and maybe I was lucky to get that first, second, third, fourth, fifth and now sixth flying job, I dont know, could have just been right place and right time.
Good luck either way. Think you need it either way in this industry, especially now and how it seems to be going.