I'm in a similar position to you, although several years forwards. I've just finished my A levels. I've always been used to getting near to full marks in all my tests, and have never received anything other than an A*, and my parents just expect it from me.
Yet in my A level further maths, I might not have even got an A- a combination of two (severe) really annoying silly mistakes means I'm now on lower marks than everyone I talk to. It's a hig deal because I need an A to get into Cambridge (otherwise it's Durham) and I was terrified of how my parents will react if it does turn out I get a B and end up missing my Cambridge offer (I'll find out in August).
I just explained to them exactly how it is. They weren't happy, but are being supportive about it now. Just have confidence that they'll care enough about you to not be to mad