You're going to have a mostly uphill battle.
There are two main reasons for this, and I hope that explaining them, you will see a path to get what you want, though it will not be easy.
The first problem is that your parents are right. Doesn't matter if you think they are, or I think they are. As your parents, they are right to push you to excel. From the time you were born until they die, they will always push for you and want for you, what they think is best. As you get older you get more of a say in that (how much depends on culture, of course) but one thing will never change. They are right.
The second issue is that when you lobby to do less work, your automatically going to come off as lazy, uninterested, spoiled, or some such negative adjective. Doesn't mean it's true, but there is no way, ever, parents or other people to advocate doing less, and not come off like your trying to avoid hardship.
Now, what that does give you is a little bit of wiggle room if you can figure out what it is you want, and what your parents want. It also can have a lot to do with if you have burnt your bridges yet as a teen, and what worked for your grandparents and parents.
First, acknowledge that their goal is reasonable and valid. You may not like it, but you can achieve it. If you can not, then perhaps you should be studying over the holiday (more on this later) though perhaps the method of study should change.
Second, acknowledge that their "penalty" is reasonable and valid. Again, you don't have to like it, but there is nothing wrong with what they are saying or doing.
Third, address their larger goal, and validate it. "I know you want me to do well in life, and that's why you're being hard on me."
Fourth, use the larger goal to accept and modify the current smaller goal and the penalty.
An example -- Make sure to modify it for your situation and not just read it off to them.
Mom, Dad, I understand that you want me to do well in the exams. I agree that if I work hard and study that I should be able to get a 96% or better on those exams. And I understand that if I don't you want me to study over the holiday. I do understand that you are trying to do what is best for me, to set me up for a good, well-educated life. However, there are a few things that I would like to talk about.
While I agree that 96% is a good overall goal, I am better in some subjects then I am in others. Is it possible that we could use that 96% as an average instead of on every test? I will still do my best, but I would like to reduce the stress of having to worry about ruining my holiday by getting a 95% in math, just because I am not as good in it as I am in English.
I also understand that if I fail to get a 96% that you want me to study over the holiday. I am going to try as hard as I can, but if I fail to get a 96%, then instead of studying over the holiday, can we discuss getting a tutor or other ways I can focus more on that class. To be honest, if I get a 94% it's not because I'm not trying, so maybe I just need additional help in that subject. Maybe we can use some time this holiday to find a tutor should I need one.
Lastly, while I know you are well-intentioned, the school holidays are supposed to be a time for me to relax and unwind, as well as work on my social skills with my peers. To be successful in life I need both a good education and a good set of social skills, one is of no use without the other.
As an aside, I know what you are trying to do by pushing me to succeed, but it's having the opposite effect. Whenever I go to study, instead of focusing on the subject I focus on how I am going to screw up my holiday. I understand your rules, but I need help with this. I can't focus. Can you just reassure me that no matter what happens, that we will still get to have some fun this holiday? That would go a long way towards helping me reduce my stress level a bit.