In Japan, senior high school is not part of compulsory education. To enroll in a senior high school, the student must have received the equivalent of a Japanese elementary and junior high school education of nine years in a foreign country (schools for Japanese children do not apply) and pass the entrance examination.
If the student wishes to enroll in a public senior high school in Osaka Prefecture, you should consult with the Osaka Prefectural Board of Education (group responsible for senior high school education, tel: 06-6944-6887). If the desired high school is a private school, please contact the school directly.