Unpaid or Leave without wages are to be treated with extra ordinary care and must be with special approval for specific geniune reasons only or else will lead to indisicipline.
You may call it extra ordinary( long) leaves (i'm not referring to occasional 1-2 days LWP) should be approved through various level and final sanctioning should be with the higher authority. There should also be rule that during the life time how many such leaves can be sanctioned. For reasons like higher education etc. you may have Sabbatical if appropriate where only lien is maintained and the employee will not get any other benefit while in Extra ordinary leave or Leave without wages employee will be entitled for certain benefits like medical, insurance etc.
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Taking approval on any leave ( Paid or Unpaid) is should be follow. The authorize person need to manage shift or work plan for team as well, and a sudden leave of an employee cause problem sometime. According to me, if leaves are planned then it should be informed to the manager or to the person who is authorize to approve or reject leaves. Leaves will be consider as Unpaid leaves or LOP if an employee doesn't have leaves in their account.