In 1917, Konstantin Treffner, H. Treffner's first cousin once removed, became the director of the school. During the German occupation of Estonia in 1918, the school switched to teaching in Estonian. The same year, Hugo Treffner Gymnasium and Tartu Alexander Gymnasium were merged and in February 1919, the school got a new home, where it is situated to this day. During the merge, the school gained the inventory and library of Alexander Gymnasium. In the Estonian War of Independence, 230 students of Hugo Treffner Gymnasium were drafted, seven were killed in battle.