It is perfectly true that there are many airline pilots who don't have particularly good educational qualifications. It's also true that all of us who aren't academically well qualified (yes, I include myself) were bloody lucky to get in to flying at a time when, or by a route where, these things weren't as important as they are now.
If you are at school and are hoping to begin pilot training soon after you leave there or university (for which you'll need good grades anyway), you will do yourself no favours by not doing the best you can to get good grades. Many of you will be aiming for one of the various CTC schemes, or one of the few part-sponsorships that exist, or the OAT APP scheme. All of these require good qualifications from their young applicants, as does the direct-entry route into many airlines. If you've been around the block a few times and have a history of work, academics are less important - but not unimportant!
Don't make it unnecessarily difficult for yourself; do your very best to get the highest grades you can at school, college or university.
Scroggs