Hello Aspirant

You are eligible for the science stream in 11th grade only if you scored 75% above aggregate in your 10th exam. So, if you have scored that much, you can do 11th with science, but you will have to take admission in 11th class again and do your 2-year of schooling again.

You can not directly do 12th with science after doing 12th with arts. In order to pass 12th with science, you will have to, as mentioned above, take admission in 11th class. Even this will only happen if you had 75% above aggregate in 10th Board exams.