All The Best as you have a dream to become an IAS . I suggest you to take B.Sc as there is science stream in your class 11 and 12 . You have many opportunities as you can do M.Sc or PhD after doing B.Sc . You have an opportunity if you fail to achieve your dream of an IAS officer . If you don't get selected for IAS then you have a chance of becoming professor .
Dream to become an IAS but have an other option also in your career . You should work very hard to become an IAS . But concentrate on your degree as Degree is most crucial to become an IAS as without a degree you are not supposed to write the prelims exam .
Hope this helps you .
As I am myself also planning to be an IAS,the best backup plan to go for is not ba or bsc to be honest. The best plan to go for is btech course. This is because even after trying very hard you are not able to clear it, ba or bsc can't land you a job while with btech you will always have a job guarantee kind of