17-year-old Md Israfil and 15-year-old Robeya Khatun were married about eight months ago
Metropolitan Sessions Judge KM Imrul Kayes delivered the verdict along with a fine of Tk 50,000 in the presence of the accused on Thursday.
Risha, daughter of businessman Ramjan Hossain of Siddique Bazar, was a student of class eight in Willes Little Flower School.
She was stabbed allegedly by Obayedul, a worker of a tailoring shop in Elephant Road area, on a footbridge outside the school on Aug 24, 2016. She died four days later.
Her mother started a case against Obayedul after the attack, saying he had sexually harassed the girl.
Police arrested him in Nilphamari eight days after the attack amidst protests by students of the institution.