Hello Aspirant,

No, you cannot repeat class 12th just to improve your score instead what you can do is you can appear for improvement examination which is conducted in the month of June/July, or what you can do is either you can appear for NIOS or any other recognized open board and they provide the facility for On Demand Examinations and for the same you can appear either in the subject in which you have less score or for some other subjects in which you want improvement, from this you can save your year but make sure to check the applicable dates on the respective board's official website.

I hope this information helps you.

Good Luck!!