Sorry I beg to disagree with #2-3 of what HR DUDE stated.
Stating part of the article wrote on Dear PAO, manilatimes.net Jan 6, 2016 by Chief Public Attorney Persida Acosta
“a provision in the employment contract regarding lock-in period or prohibiting an employee from resigning or otherwise terminating his employment before the end of the period stated therein is valid because parties to a contract are free to establish such stipulations, clauses, terms and conditions as they may deem convenient, provided they are not contrary to law, morals, good customs, public order or public policy (Article 1306, Civil Code of the Philippines).
Based on the foregoing, your employment contract with a fixed term and containing a lock-in period is valid and binding upon you provided that you knowingly and voluntarily agreed thereto. Hence, you are bound to comply with the provision thereof in good faith. A violation of any of the provisions in the contract constitutes breach of contract that may hold you liable for damages. This is not considered unfair even if it turn out later that the terms became unfavorable to you.”
In an educational institute / setting, it is hard to find competent workers since those are already working during the school year. It will take months get the right candidates, thus the reason for setting a “lock-in” period to the contract of the employees.
anyway, everything is always based on the interpretation of the law by the lawyers and how good their argument is. my advice still, even if it is not your responsibility to find a replacement, help them find one asap in order for you to get what you want.