The 17-year-old was approached from behind as she was walking along a cut-through between Westward Ho and Littlefield Lane, which runs alongside Birkwood Medical Centre, yesterday afternoon.
The man, described as having a "deep raspy voice", sexually assaulted her, before fleeing.
The incident happened close to Orimiston Maritime Academy and The Academy, which is part of Grimsby Institute when the incident happened.
The local colleges put out alerts to parents yesterday. Initial concerns that the girl had been the victim of an attempted abduction were unfounded.
Humberside Police said today extra officers would be on patrol to reassure students travelling to and from schools and colleges in the Westwood Ho area.
The man is described as white, in his 30s, 5ft 8in, skinny and scruffy looking.
He wore a black beanie hat, grey jogging bottoms and grey shoes with white soles. His trousers were tucked into his socks.
Det Insp Rhod Troake said: “We believe this is an isolated incident, but we will have additional officers in the area over the coming days to reassure students and parents.
“We would like to thank all those who have come forward with information so far and we are continuing to investigate the circumstances.
“If you have any information, or have seen anyone acting suspiciously in this area, please get in touch.”