Dear friend ,
12 class practicals are not so tough .stay calm and confident.
I am sharing my experience with you:::
Physics:
My school lab teacher allot experiment to all students according to their choice. After that write experiment in answer sheet .Meanwhile external invigilator take viva of students. School teacher help each student in every step. Viva is not so tough. And every student get good marks in external .i got 29 out of 30 .
Chemistry:::
Experiments allot to the students  according to their choice. Meanwhile viva is taken. In viva basic reactions and chemical used in the experiment were asked.Lab faculty help students .
In short don't worry. Every student get good marks in external.
Good luck !!!!