Staff apply for leave, the office approves or rejects it with remarks, and each person's balance updates on its own.
Teachers can also apply from Teacher Mobile › My Leave Applications.
In Leave Requests, click Approve or Reject. The dialog shows the number of days and how the balance will change (for example, 15 to 13). Add remarks if you like and confirm. The row then shows the status and who reviewed it.

Staff Leave Balances lists each person's total, used and available leave. Use Edit Balance to change one, or Create Balance Rows to give everyone without a balance the same number of days; days already on approved leave count as used. Leave Calendar shows who is away each day.