Chronological summary of HTG's most important historical events:
1883 - On December 7 Hugo Treffner received permission to open his own school - progymnasium with three classes, language of instruction was German.
1886 - The school was moved to a new location at 2 Hobuse Street.
1889 - The school's curriculum switched to Russian.
1890 - The school was granted rights for an 8-classed gymnasium.
1907 - The school was granted the name and rights of a private gymnasium.
1912 - Hugo Treffner died, leadership of the school was taken over by his widow and several teachers.
1917 - Konstantin Treffner became headmaster.
1918 - The school's curriculum switched to Estonian.
1919 - In Februrary, the school was moved to its present location on the corner of Rüütli and Munga, becoming the largest school in Estonia at the time. A section of the former Alexander Gymnasium was merged with the new schoolhouse.
1933 - Celebrations for the 50th anniversary of HTG took place. The festivities were attended by Konstantin Päts, the then head of state of Estonia.
1940 - HTG was renamed Secondary School No. IV.
1944 - The former HTG was merged with Tartu I gymnasium, forming the new Tartu Secondary School No. 1.
1954 - The school was changed from a boys' school to a mixed-gender school.
1958 - The school was renamed Anton Hansen Tammsaare Tartu Secondary School No. 1.
1961 - The first specialised maths class was opened in Estonia.
1964 - A specialised physics class was opened.
1967 - The first five-school competition was held. The participants were A.H.Tammsaare Tartu Secondary School No. 1, Carl Robert Jakobson Viljandi Secondary School No. 1 and Nõo Secondary
School. Miina Härma Tartu Secondary School No. 2 and Tartu Secondary School No. 5 joined the competition later (1967).
1969 - The specialised maths and physics classes were merged into a science class.
1976 - Forms 1-6 were moved to the "Little house" at 28 Lai street (present day Kesklinna School).
1984 - The 100th anniversary of the founding of Hugo Treffner Private Gymnasium was celebrated with a conference in the Tartu University assembly hall and a student concert in Vanemuine concert hall.
1989 - Hugo Treffner's gravestone at Uue-Jaani cemetery in Raadi was restored with support from school alumni (sculptor Mati Karmin). The first specialised natural science class was opened.
1990 - The school's historical name, Hugo Treffner Gymnasium, was restored.
1994 - A course system was introduced.
1995 - Admission to form 7 ended.
1996 - A humanities class opened.
1997 - A statue of Hugo Treffner was opened to the public in park by the Emajõgi river (sculptor Mati Karmin).
1998 - On November 16 a fire broke out in the schoolhouse.
1999 - Renovation work began.
2000 - The school was moved back to the partly renovated schoolhouse.
2001 - The new gym was opened.
2002 - Renovation work was completed in December.
2008 - On December 5 a museum of HTG was opened to coincide with the school's 125th anniversary.
2011 - In November, HTG became partnered with UNESCO.
List of HTG headmasters:
Hugo Treffner 1883-1912
Vladimir Usbenski 1913-1914
Nikolai Sahharov 1914-1917
Konstantin Treffner 1917-1940
Johannes Valgma 1940-1941
August Raielo 1941-1944
Karl Maasik 1944-1950
Juhan Truus 1950-1953
Elmar Loodus 1953-1956
Allan Liim 1956-1970
Uno Langer 1970-1975
Laine Raudsepp 1975-1987
Helmer Jõgi 1987-2001
Ott Ojaveer 2001- I