Offshore Islands Development Act Article 12-1

A teacher who has entered into initial employment to teach at a senior secondary school, or a lower level, on any offshore islands should serve at least actual six years in such a position before the teacher may apply for a teaching engagement on the mainland Taiwan, in order to guarantee the right to education of students who inhabit on outlying islands.The exact years of service described in the preceding paragraph should deduct the service seniority in a position retained without pay, in addition to the service seniority during parental leave without pay or a position retained without pay due to being conscripted. The service seniority is limited to actual years of service at the school the teacher is currently serving.
中文原文 離島建設條例第12-1條
為保障離島地區學生之受教權,離島地區高級中等以下學校初聘教師應實際服務六年以上,始得提出申請介聘至台灣本島地區學校。
前項所謂實際服務年限,除育嬰或應徵服兵役留職停薪者外,應扣除各項留職停薪年資;以實際服務現職學校年資為限。

English translation for reference only; the Chinese text prevails. Not legal advice.