A search is underway in Moore Park in inner Sydney for a man who sparked a lockdown and evacuation at Sydney Boys High School after he was challenged by a teacher.

The alarm was raised about 2.20pm today after a school staff member spoke to the man, who was carrying a grey shopping bag, and became concerned by his behaviour.

The high school, on Cleveland Street in Moore Park, was placed into lockdown for a short period of time as a precautionary measure.

NSW Police initially said they had received unconfirmed reports the man was armed but later clarified that no firearm was sighted.

The man was described as being 20-years-old and 175cm tall with short dark hair. He was wearing light blue jeans, a white shirt and a pair of hiking or work boots.

There were no injuries in the incident.

Cleveland Street was temporarily closed in both directions between Anzac Parade and South Dowling Street.

There are 1200 students at the school.

Police are continuing to patrol the area to ensure students are able to leave the school safety.

The evacuation and lockdown were just a precaution, police said. (9NEWS)