CA

Yes, it is possible, but it would be too hectic for you to concentrate on two subjects together.
CA course can very well be done along with your commerce graduation. However, you will have to wait till you complete your HSC Commerce with minimum 50% marks in aggregate. B.Com and CA course can be done simultaneously. And, in fact there are thousands of students who opt for the Company Secretary / Chartered Accountant / Cost Accountancy course alongside the graduation.
Do not neglect graduation studies at all. Your Commerce graduation is a solid foundation for your post-graduation ahead in life. It happens that since the professional courses like CA, CS, CWA require lot of hard work and time, students neglect the graduation studies. This should not happen with you my dear, is what I feel.
Therefore, score good marks in HSC Commerce, and then you can apply for Company Secretary Course and do graduation and CS course simultaneously.

To give a simple overview, a student should go for CA if the student wants to make his/her mark in accounting initially. Because later at the last module of the exam, you will have so many subjects which will allow you to choose any domain of finance you like. But first you need to be exceptionallygood in accounting to clear the first and second module.
A CS is someone who will take over the corporate governance of the company. S/he will be a master of corporate laws and practice and will be able to act as a direct advisor to the board of directors of the company. To pass CS, you need to have special liking for Law and how it works in company perspective
hope it's very clear what you want to take.

Course wise difference is already known to all.
Basically, a chartered accountant is someone who has expertise in Taxation, accounts and auditing. On the other hand, a company secretary is one having expertise in all the laws, rules and regulations.
Numbers/taxes or law/company affairs
Both have different work fields and opportunities. A time was there when CA was more remunerative then CS. But the change in time and induction of new laws have widened the scope of CS too and therefore more or less they both stand at equal levels in case of employment.
However in practice, we need to remember that anybody and everybody can approach a chartered accountant for basic accounting or Taxation work but same is not the case with company secretary.
CA is for the masses.
CS is for the classes.

You can only begin preparing for CA, once you begin with your 11th grade (I am assuming you will opt for commerce stream).
You need to focus on your 10th standard subjects as of now as there is hardly anything you can prepare in 10th standard that is going to help you in your CA exams (except maybe a little bit compound interest, simple interest).
While you are in your 11th and 12th grade, you need to just put in some extra effort in certain subjects that will further help you sail through CA common proficiency test (CPT). This is considering the fact that there is a change in examination pattern from objective to subjective.
In 11th grade, just keep your basics clear about subjects like accounts, statistics, mathematics and economics.
In 12th grade, you are required to put more emphasis on certain subjects namely accounts, economics (Micro+Macro) and mathematics (calculus). If you do have time to spare, you can go through the basics of Indian Contract Act. Lastly, do not forget about class 12th exams and devote time accordingly.
After you are done with class 12th exams, you have 3 months before your CPT exam (give or take a week). You just need to plan a routine for those 3 months and you will get through.