If you do go on to marine science or something like that then you will find that most people over there aren't particularly good at maths / physical / analytical chemistry. You will need some bio-chemistry / organic chemistry but doing well at that will surely distinguish yourself from the 'I just wanna swim with dolphins all day' hippy-kids.

If you are talking about school in the US then you have to do a whole degree before you can apply to study medicine. So I don't understand why you keep talking about what degree to do or what major to do or whether you should double major... The point is that you HAVE TO do a degree in something NOT MEDICINE before applying to medicine. And you surely don't need to decide whether you are going to apply to medicine or not, now. Just nod and smile for the grandparents, nod and smile.

You would also have to sit a test for entry to medicine. This gives you some idea: https://www.khanacademy.org/test-prep/mcat Universities don't specifically prepare you for that exam, but some people do select their undergraduate degree with that exam in mind. They have recently changed it (broadened it) so it has some psychology / sociology stuff in it. To encourage people to take social science degrees as undergraduates if they want. So... You could take a social science degree if you wanted... In something like psychology... Because you are interested in animal behavior, or whatever. You could totally do that.

The whole point of making you do a different degree first is that a lot of people start out thinking that they want to do medicine and they end up not wanting to do it. Or, conversely, a lot of people start out thinking that they don't want to do it, but it starts to seem appealing to them as they get a bit older. You simply don't have to decide now.

Some people do really well in High School and they fall apart at University. Don't cope with the freedom. Decide they would rather party, or can't work without parents / teachers standing over them cracking the whip. Other people don' do so well at High School but they really flourish at University. Because you have more choice to do what you are interested in and are so much more likely to fall in with other students who are similarly interested (so to form good social / study groups). I did really badly at school and was shocked to find myself turning into an A student at University. So... You never know what will happen for you

The system where I am largely takes students as undergraduates - and I think it is not a good idea. I know a lot of people who start out being happy to get offer of place and they end up dropping it when the clinical stuff starts because they simply don't have the emotional maturity to be dealing with such things at their age. They drop it... Then after completing PhD's in different fields... They have a longing to return. I think the US system is much better for making people wait....

> I dunno, maybe I can double major or just think about other ideas than following a line where some of my passion is missing. And also, I hate to admit, but most of my sciences are in the B- - C- range (just semester grade averages). I'm going to feel like a failure.

Finish up high school as best you can and try and get into studying something that sounds interesting to you. A lot of people change their minds over the first year or two of their degree as they get exposure to different things. You won't get into medicine with grades like that, but it is next to impossible to try and force your mind to do something that you aren't interested in solely in order to make others happy.

I would hope that your parents / grandparents would be happy to see you enjoying what you are doing and doing well in it. Even if it isn't medicine.