In 2009, Kajang High School had 1306 male and 113 female students, making a total enrolment of 1419 people.

The establishment of this school was championed by Taukeh Ng Bow Thai, the late Raja Alang, Mr. Arumpalan, Mr. V. Sabapathy, and Mr. Gan Boon Teik.

When the Japanese invaded Malaya, the school building was taken over by the Australian army as a place to stay.

The number of students this year has risen to 1143 compared with an average of 300 people in the days before World War II.

In the same year, a workshop for vocational subjects (carpentry, metalwork, study and analyse the electrical power) was completed.

Additional rooms were built in the early months of next December 1964 for teaching Industrial Arts.

It was called Low Ti Kok, named after one of the founders of the High SMK Kajang.

YB Encik Abdul Rahman bin Haji Talib (Minister of Education Federation of Malaya) officiated the opening ceremony.

Construction of the new hostel building began in 1972 and was completed in 1973 and was officially opened on 1 July 1975 by Mr. Chan Sang Chang on behalf of then Prime Minister of Malaysia, Tun Dr. Mahathir Mohamad.

However, it caught fire in April 1998 and has since undergone renovation and repairs as of 2015 is still operating to cater to students from Selangor and other states.

The school motto is Berilmu, Berdispilin and Berjaya meaning Knowledge and Discipline empowers Success.