The deceased was identified as Hossain Ahmed, 18, son of Samir Uddin, a resident of the area and also a second-year student of Bianibazar Govt College.
Locals said Suman Ahmed, son of Muhib Ali of the area and his accomplices were beating a minor boy in front of Nidanpur Government Primary School in the area around 3pm Monday.
The culprits became furious on Hossain when he tried to prevent them from beating the minor and, at one stage, they thrashed Hossain, leaving him severely injured.
The injured was rushed to Sylhet Osmani Medical College Hospital and later shifted to a city hospital where he succumbed to his injuries on Wednesday.
Officer-in-charge of Bianibazar Police Station Abani Shankar Kar said they are trying to arrest the accused after a case was filed with the police station on Wednesday.