Hello , 

As your query is you want to know that if a student from 12th standard stream science fails from the CBSE board then whether the candidate has to appear for the practical re-examination. I would like to tell you that the marks of practical has nothing to do with the the marks of the candidate scored in 12th boards both of them have different bases and are marked on different scales. So as per your situation is concerned the student has not to give re-examination of practical all that student need to do is to apply for the re examination for improvement examination for or his or her 12th standard. 

I hope your query is solved.

Thank you.