The contract names a large fine for leaving early — often equal to a month or more of salary, sometimes denominated in USD. Teachers are told they cannot legally leave without paying. These clauses are generally unenforceable, but the employer enforces them simply by withholding your final month.
Look for 'breach penalty', 'liquidated damages', 'notice period' amounts. Note if it is written in USD — a sign it targets foreign staff.
Poster is weighing a 'midnight run' — leaving without notice — because the fine makes normal resignation cost them a month.
¥25,000 is close to two months' pay on a ¥14,000 salary.
Deduction used as leverage after the teacher raised the missing insurance.