Nusrat Jahan Rafi, 19, died in April in Feni, a small town some 160km (100 miles) outside the capital Dhaka.
Those convicted of murder included the headteacher Nusrat had accused of harassment and two female classmates.
Her murder shocked the country and led to a series of protests demanding justice for Nusrat.
The trial has been one of the quickest in a country where such cases usually take years to conclude. Prosecutor Hafez Ahmed told reporters it proved "that nobody will get away with murder in Bangladesh".
However, the quick conclusion of the case did little to ease her mother's agony on Thursday.
"I can't forget her for a moment," Shirin Akhtar told news agency Reuters through tears upon hearing the verdict. "I still feel the pain that she went through."