BSNL leave rules are attached. Kindly go thru study leave carefully.
However it shall be appropriate to obtain leave rules/study leave from your company and go thru it carefully.
The union/HR/Personnel/admin in your company can guide you how to obtain the rules applicable in your company or you can obtain these thru RTI.
You may approach the line management, HR,eave sanctioning authority etc and seek help.
Usually establishments are afraid that extra oridnary sanctions may become a precedence.