Ainsley it is not your 'fault'. It takes two to make a baby. please do not feel guilty any more about this, it happened, you did not plan it, it took two and now you need to move on and do what is best.
You alone can decide what is best, with advice.
I agree with BarbarianMum.
As far as I am aware the issue of relatives being contacted to look after babies is if they are taken into care, not if they are relinquished. As far as the father goes does he want to be involved? If not then you do not need to think too much about his involvement, IMHO.
You age of 15 was mentioned, maybe you would you be 16 by the time the baby was born?
You are a child legally but as you are having a baby you are doing something which is quite 'adult' so you are being forced to make decisions and choices which normally affect adults.
I think you need to (as much as you can) forget about the comments of cruel and stupid classmates. Who knows many of them may have had sexual relationships etc. Who are they to judge you - either may. Unfortunately, you are in a difficult situation but you will at least find out who your real friends are.
I know that is very young. I do understand your desire for continuing with school etc. It most definitely is not selfish.
Please start to be kinder to yourself, you are not selfish or bad, you and this young man just made a mistake, which has very large consequences. Whichever way you go, it is your choice and you need to feel positive about yourself in all this. Whether you give your child up for adoption (and make a childless couple incredibly happy in the process) or ask your parents to help raise him or her (which will make them happy, presumably) you will be doing an amazing thing for your child.
But what would be the right thing for you, only you can work that out.
I wonder how old your parents are? Are they 40s, 50s? I just adopted a child aged 3 (with my dh) and I was almost 50 when I adopted. We also have a daughter of 10. Our son was not relinquished but taken into care, so not the same situation at all. But my point is I am an older mum (though often told I do not look my age!). You may feel your parents are older but actually if your parents are 40s they are not that old. Or even 50s would not be very old, if that is a concern.
Would your parents raising your child mean they had to pretend to be the mum and dad? Surely not! I believe they could raise the child as 'kinship carers' and the child would know you were their birth mum. Of course if they were providing the care day by day the child would think of them as their 'parents' but would also know the truth and would probably think that is all pretty normal. Not strange or odd and most of all not secretive (which is generally very destructive IMHO).
Good luck, please keep talking to us if it helps.