Hello,
.
About which standard are  you talking about? If you are talking about +2 then you can only change the stream after completion of 10th and 11th but not in between on 12th. If you are talking about graduation, then there are no Chances to change the stream if it's been 2 to 3 months of admission. While the time of admission, there will be option to changes the stream till a Month and later you will have no chances.

Hello,

Since you have already completed class 11th so i am assuming you have already done registration for boards and in that case you can't change your stream in that school.Now, you only have the option of studying science stream with any state board.
You can still try to consult to your school headmaster regarding the matter hopefully he might help you.

I hope this helps.