Bagerhat District and Sessions Judge Khan Mizanur Rahman on Monday also fined each Tk 10,000.
The convicts would have to serve one extra year in jail, if they fail to pay the fine.
The court, however, acquitted three of the accused as the charges against them could not be proved.
All the accused were present in court when the verdict was delivered.
The slain BCL activist, Sheikh Arif Hasan Raju, was the son of Nur Islam Sheikh of Jatrapur village of Bagerhat Sadar.
Sheikh, unhappy with the sentence, said he would appeal to a higher court for a more severe punishment.
The suspects had planned the murder over a land dispute, he said.
According to the case details, Raju had been beaten to death by his party rivals on Sep 4, 2006, after being picked up from a BCL meeting in Jatrapur Union.
Eight of those convicted – Hossain Sheikh, 21, Sheikh Faruk, 33, Nuru Sheikh, 40, his brother Abdus Sattar Sheikh, 54, his four sons Razzak Sheikh, 23, Lavlu Sheikh, 21, Nazrul Sheikh, 28 and Shawkat Sheikh, 30 – are from Jatrapur village.
Abdul Kader Sheikh, 42, his two sons Nazrul Shiekh, 28 and Ziarul Sheikh are from Afra village, while Sheikh Tipu Sultan, 22, hails from Chanpatala village.