If your school is a centre for Madhyamik Exam, your HM is the only authority who can allow you to take leave during that period as @mihiracharya said. So it is all up to your HM. Keep in mind that invigilation to MP is a bounden duty and you should reschedule your journey, if your HM is not convinced to allow you to take leave during that period. By the way, you had had enough time to do that as Madhyamik Exam was scheduled much earlier.
