1903 – Tallinn Elementary School No 2 is founded at 6A Poska Street.
1917 – The school is renamed Tallinn Elementary School No 21.
1923-24 – A new schoolhouse is erected at 6 Raua Street. Architect: Artur Perna.
1940 – On 21st June the soldiers of the Signal Battalion of the Estonian Defence Army based in the schoolhouse resist the siege by the invading Soviet Army.
1945-46 – Secondary school level courses of study are introduced.
1950 – First 14 students finish secondary school.
1958 – The first specialised music class opens.
1961 – The first art and English specialised classes start. The first annual school concert is held in the Estonia Concert Hall.
1965 – An annex building for primary students is completed. Architect: Heili Härmson.
1966 – The first educational institution in the Soviet Union to introduce a shorter five-day week.
1967 – Construction of the new swimming pool and gym is completed.
1986 – The first Week of Art and Culture is held.
1987 – The school gets its flag and a graduation ring, designed by Andres Ojari.
1988 – The first student exchange programmes are conducted with the Dalton School, New York, Kingswood Oxford School, CT and Mariner High School, WA, USA.
1991 – The first issue of the school newspaper ’Lennuk’ (’Airplane’) is published.
1993 – The school joins an international initiative for promoting healthy lifestyles in schools.
1994 – The exchange programme with Lahti school in Finland starts.
1995 – The School Student Council is established.
1996 – A joint co-operative body of Five Schools (G5) is initiated by Tallinn Secondary School No 21.
1996 – The school joins The Baltic Sea project.
1997 – The first computer classroom is equipped. Following the students’ initiative National Defence courses start.
1998 – The school is renamed Tallinn School No 21.
1999 – The school gets its school cap. Designer: Jan Garšnek, alumnus ’50.
2000 – The school is awarded the title of ‘A European of the Year’.
2001 – Renovation of the school building starts. Lessons are temporarily transferred to other schools.
2002 – A three-school project of humanities commences. The participants are from Hugo Treffner Gymnasium, Pärnu Sütevaka Gymnasium of Humanities and Tallinn School No 21.
2003 – The school celebrates its centenary with the opening of a newly renovated building designed by architects Raul Järg and Toomas Korb.
2003 – A book about the history of the school ‘A Hundred Years of the School Bell’ is published.
2005 – The school is awarded a certificate for outstanding extracurricular activities.
2006 – Transition to the online system ‘e-school’ is completed.
2007 – The tradition of staging musicals in cooperation with German students from partner schools is established, ‘Hans and Grethel’ being the first production.
2009 – Student initiative leads to the introduction of school waistcoats as an element of uniform.
2010 – The 50th annual school concert is held.
2011 – The 25th anniversary of the Week of Art and Culture is celebrated.
2012 – The school is awarded a certificate for outstanding learning environment.
2013 – Tallinn School No 21 celebrates its 110th birthday.
2013 – A film introducing the school is released.
2014 – The school is presented a certificate for ‘Innovative Act 2014’.
2015 – The school is presented a certificate for ‘Innovative Act 2015’.
2016 – The school is recognised as an ‘Innovative Learning Establishment 2016’.
2016 – The is awarded ‘Best Cooperative Project 2016’.
2017 – The school is recognised by the city of Tallinn as School of the Year 2016.
2017 – The school is awarded a certificate for its digital activities by the Ministry of Education and Research.
2017 – The school is awarded a certificate by Education in Tallinn for ‘Education Act’ and a special prize for participation in the Flower Festival.