This question in one form or another turns up frequently. It is important to remember a few things when it comes to recruitment, in any industry and that includes aviation.
Firstly what is the employer looking for ? What experience do they require ? What qualifications do they stipulate ? Remember that for many jobs ( and particularly for attractive jobs) there will be a very high volume of applicants for each vacancy. With that in mind it is impossible to interview each and every possible applicant. The criteria is laid down to whittle down the number of applicants to a more manageble level. It is possible that you could offer a substitute qualification or level of experience if you do not otherwise meet the employers requirements, but in all likelyhood you are unlikely to be considered.
If an employer is looking for experienced pilots then they are more likely to pay attention to the relevant levels of experience, (hours on type, previous employment, type of operation, professional qualifications etc.) than they are to the educational certificates obtained 5,10,20 years ago. However the "catch 22" is how do you get into this position without having first overcome the initial qualifying criteria further down the food chain ?
The situation with airlines is that if you haven't got the required licences, then don't bother wasting their time. You may think they are interested in your future plans, and the letter they
might send you saying "good luck in the future" may give that impression, but in reality without current and adequate professional qualification they are not interested.
At the next level up, licence holders with low levels of experience, you are in plentiful company. In other words there is no shortage of wannabees. For those companies that are prepared to consider employing low hour entrant pilots, the supply is abundant. It is here that the importance of educational qualifications comes into its own. No one should be under any illusion that there are plenty of applicants with good qualifications at high achievement grades. As I already mentioned, those qualification subjects, quantity and grades achieved are initially used as a way of simply reducing the application pile. For those that are successful in being selected for interview, the qualifications show a level of attainment, application and attitude that reflects on what the employer is looking for in the applicant.
It is true to say that good grades in secondary subjects are probably better than weak grades in primary subjects. The old favourites of Maths and Physics are always going to have an edge on other subjects, however excellent grades in say English Geography and Biology, are probably going to impress more than a candidate turning up with a C in Maths and a D in Physics. Unfortunetaly for the former there are a lot of candidates who turn up with good grades in good subjects, and clearly they will always have an advantage.
In summary, it is a game of numbers. There are plenty of people at the bottom of the pyramid, looking for the jobs near the top. Those with the best qualification criteria (or the highest grades/numbers) are most likely to fast track themselves to the next level. The higher you rise in the pyramid, so the criteria tends to become more experienced based. Even so there are jobs higher up the food chain where the educational criteria may once again become relevant.
For what it may be worth, you rarely see people at interview that are anything other than excellent candidates. They are usually articulate, well groomed, well educated and have a solid enthusiasm and knowledge of the position they are applying for. This is because the selection criteria from the information on their application forms and C.V's weeds out the vast majority of applicants who are anything else or anything less. Of course a few get through, but in reality the interview stage is used to make a final selection from a very strong group of applicants.
There are plenty of people who will tell you how they got to where they are (myself included) without anything more than average educational qualifications or indeed any at all. There are methods of achieving a career path that can sometimes overcome the normal requirements. This was certainly more the case in the past, and in times of supply shortage, than is the case today. Luck still plays a significant part in the process, and that is not necessarily relevant to your educational qualifications. However it would be very naiive to think that your chances of success in achieveing a fast track introduction into an airline career in todays world, would be best achieved with anything other than a good solid set of educational certificates to underpin the professional licences you are also required to hold.
I say this as a senior airline captain who has been involved with airline pilot recruitment. I also have an 18 year old son who like you has his heart set on a career as a professional pilot.
I hear i'm competing against the best aviation has to offer
Trust me, when it comes to low hour pilots you are not competing with the "best aviation has to offer". You are competing with other low hour pilots who are trying to impress that they are the best educated, and presented individuals that they can be.