Sydney Boys High School was put in lockdown after reports of an armed man on school grounds on Wednesday afternoon.

Students were told to stay in their classroom and hide under their desks while police initially looked for a man thought to be armed on school grounds.

Police and emergency services were called to the school on Cleveland Street in Moore Park about 2.20pm by a staff member after a suspicious man was seen on school grounds and was challenged by a teacher.

But a police investigation found the man was not armed. Reports suggest he was holding a bag and a coke bottle.

ABC reporter Brigid Glanville said her stepson hid under his desk while a policeman was outside the room.

My step son at Sydney boys High who is ok, was told to stay in the classroom and hide under the desk. A policeman was outside the room.— brigid glanville (@brigidglanville) August 12, 2015

A student told Fairfax Media that a year 9 student found a man crouching in the bathrooms on school grounds with a bag.

The student said everyone was evacuated immediately but they were let back into the school shortly after.

A police inspector at the scene said the man ran from school grounds and hasn't been seen since.

Officers will continue to search for the man, he said.

No one has been taken into custody and there is no suggestion the man was carrying a gun.

"The school was placed into lockdown for a short period of time until the safety of all students and teachers could be assured," police said.

"At no time was a firearm sighted and there are no reports of any injuries," police said.

Students were evacuated to a nearby oval but were allowed to return to their classrooms about 3pm.

Police have launched a search for the man in Moore Park.

He is described as being about 20 years old, and 175 centimetres tall with short dark hair.

He was wearing light blue jeans, a white shirt and a pair of hiking or work boots.

He was last seen carrying a grey shopping bag.

Sydney Girls High School, which shares some ground with its brother school, was not affected.

Cleveland Street has been reopened after emergency services closed the road in both directions between Anzac Parade and South Dowling Street while police conducted their investigation.

*/ ]]>