A 16-year-old boy was beaten outside Redbridge College by a gang who threatened to go to his house and stab him if he told police.
Officers are appealing for anyone who saw the attack on January 16 to come forward.
A Redbridge Police spokesman said the victim had left college, in Little Heath, when between six and eight men wearing hoodies surrounded him.
He saw a knife but it was not used in the attack that left him with bruises to his face and body.
The teenager was treated at A&E at King George Hospital in Barley Lane, Goodmayes.
He told officers the group were friends with another boy who had robbed him in December.
Two men have been arrested on suspicion of witness intimidation and grievous bodily harm and bailed until March.
Police are still looking for the remaining suspects.
Anyone with information should call Redbridge Police on 101 or Crimestoppers anonymously on 0800 555 111.
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