The 17-year-old victim suffered a serious facial injury in the unprovoked attack which happened near the fountain and pond at around 11.40pm on Friday.
Officers believe a group of four or five male youths were involved in the incident.
The suspects are described as being 17, one of them was 5ft 9 in, with dark hair, wearing dark t-shirt and shorts. Another was carrying a white bag.
Police have appealed for witnesses.