A teenage boy suffered serious facial injuries while two of his friends were also injured after being assaulted at a travelling fairground at Hook Road Arena.
The attack happened on Friday, April 4, when the three teens attempted to intervene after seeing a group of 15 males harassing either two girls or a boy and a girl at about 8.20pm.
But the group turned on the three boys instead.
One of the victims, aged 14, suffered serious facial injuries to his jaw and cheek.
The other two boys, aged 14 and 13, also received cuts and bruises in the assault.
Detective Constable Karen Scholey from East Surrey CID praised the boys for their "bravery and courage" and appealed to anyone with any information to come forward.
She said: "I need to trace the two people who were being harassed by the group prior to the assault.
"I would like to stress that these two youngsters are not in any trouble but I believe they may hold vital information on the assault and would like to speak with them urgently.
"The boys who intervened showed bravery and courage but as a result they suffered not only physical injuries but an experience which will no doubt stay with them for some time.
"Therefore I urge anyone who can help with the investigation to come forward and share information with police."
Call police on 101 quoting reference 45140026643 or call Crimestoppers anonymously on 0800 555 111.