The teacher was arresting during protests that led to the overthrow of former president Omar al-Bashir.
It was the first time courts have handed down convictions over crackdowns on demonstrations in the months before and after Bashir was toppled in April.
Thirteen defendants were sentenced to prison terms and a further four were acquitted in the verdict, which could face several stages of appeal.
The death of teacher Ahmed al-Khair in the eastern town of Khashm al-Qirba became a rallying point during 16 weeks of protests against Bashir’s rule.
Khair’s family said that security officials initially claimed he had died of poisoning, though days later a state investigation found he had died of injuries from beating.
Hundreds of people rallied outside the court in Omdurman where the verdict was delivered on Monday, some waving national flags or holding pictures of Khair.