You just need to see it from your parent's point of view. All your parents want is what's best for you. You are probably too young to see that. I can't really talk because I didn't really put much effort in at school, but I do think it's definitely worth it. It's either that or leave school and get a job on minimum wage*, and do that until you die* with very little time for riding*.
Like everyone has basically said, don't do it for your Mum or anyone else, but bear in mind that a bit of work at this stage will go a long way. You never really hear of people who regret working hard at A-Levels, but you hear of loads of people who regret not working hard enough at school. But perhaps you should have chosen A-Levels that were less dull :$
*Probably