This study developed and validated the indicators of school internationalization in elementary and secondary schools in Taiwan to identify their level of internationalization and assess how the same has been implemented in these institutions. The data for the study was obtained from focus group interviews with 14 school internationalization experts and 176 school principals. Subsequently, 145 sample schools were recruited to complete an online questionnaire as part of a formal survey. The 20-item indicator scale regarding internationalization goals, campus internationalization, personnel internationalization, administration internationalization, curricula internationalization, and international partnerships proved suitable to orient elementary and secondary schools in Taiwan toward establishing international environments. Furthermore, results have important implications that the government should provide additional financial support and educational resources to reduce the teacher turnover rate and increase the willingness of schools in Taiwan to participate in international affairs.