It admits upper-secondary students (mathayom 4–6, equivalent to grades 10–12) and has the largest yearly enrollment in the country.

Part of the staff and students were transferred to various cities around the country, where preparations had already taken place for the establishment of additional preparatory school campuses.

During that period, government policy favoured expanding pre-collegiate level teaching to public schools under the Department of General Education instead of being provided by the universities.

As a result, authority over the school was transferred from Chulalongkorn University to the Department of General Education on 1 July 1947.

This department, which became known as the Satit (demonstration) School, would extend teaching to mathayom 1–6 levels, and authority was transferred to the College of Education (now Srinakharinwirot University) in 1955.

Another campus, Triam Bang Pu, was established at Bang Pu Seaside Resort in Samut Prakan Province before being moved to Sam Phran District, Nakhon Pathom Province, and became known as Triam Udom Suksa Sam Phran School.

Students featured prominently in national stages, and since 1990 have continuously won medals at the International Science Olympiads.

At present, Triam Udom Suksa School educates upper-secondary students in the mathayom 4–6 levels.

[5][a] Triam Udom Suksa School is located at 227 Phaya Thai Road, Pathum Wan, Bangkok, amongst the campus of Chulalongkorn University.

Designed with influences from the De Stijl movement by Italian architect Ercole Manfredi, it is regarded as the first use of modernist architecture in a Thai educational building.

Additional courses follow the three major concentrations: science–mathematics, language–mathematics, and language (chosen among French, German, Japanese, Spanish, Chinese or Korean).

[9] Admission to the school is mainly through examinations, although there are also designated quotas for students with specific abilities and achievements.

In the 2005 academic year (the last where national rankings were publicly announced), Triam Udom Suksa had the highest mathayom 6 O-NET (Ordinary National Educational Test) average scores in the subject areas of Thai language, social studies and English language, while ranking second after Mahidol Wittayanusorn in mathematics, science and the total average.

Students regularly participate in regional and national academic competitions; the school team is usually a major player in the televised Shell Quiz programme.