Hello Aarti

I think you are talking about MH-CET, so i am answering this question considering that only.

No CET is not necessary for BSC but 12th marks matter a lot. If you score good, you have choice of choosing what to do in BSC but if you take 12th lightly you will have to settle with the circumstances. And even if you give CET as a try but have given board exams with full focus, you can still have a decent score.