Dear Raja,
If D.I./s approved in respect of M.C. resolution you may admit as per norms of the concern Institution and continue your study without hampering your normal duty i.e which you have written your application.
Before the Examination you may apply to the Secretary, W.B.B.S.E. through the Head Master With relevant papers as such as Copy of M.C. resolution, D.I/S permission, S.S.C recommendation letter, Appointment letter. Examination routine,Admission record, Forwarding letter of H.M. etc.
Then Secretary, W.B.B.S.E.may grant study Leave as a special case Under Section 28 (2)of W.B.B.S.E. Act.1963. before 7 days from the beginning of your Examination to last day of the said Examination.
Remind you if grant, it is only one time during your service life.