I guess you wanted to say "I am a secondary school student" meaning "저는 중학생(or 고등학생) 입니다" The term secondary school which is a school for young people between the ages of 11~18 is not used in Korea, instead we use "middle school(중학교)" and " high school(고등학교)" \^O^/

That is "나는 초등(or고등)학교에서 공부한다". Because 'education system of South Korea' different from that of the English-speaking world. In other words, a secondary school mean 초등or 고등학교 in Korean.