He was speaking after yet another vandalism attack on the old school buildings on Culmore Road.
The site has been plagued with vandals since the college shut its doors to pupils
Councillor Hassan said: “The former Thornhill College on Culmore Road was the subject of yet another break-in by vandals this week .
“Once again those behind the attack who came prepared with equipment to pull off shutters and then go on a wrecking spree within the buildings, thousands of pounds has been spent trying to stop the vandalism.
“This former school complex has been a magnet for anti-social activity over the past year. It has been subjected to numerous arson attempts.
“Parents need to know where their children are and advise them to stay away from this precarious site.
“These buildings and lands are in a very dangerous state and are not a playground or safe place to congregate.”
There are currently proposals for the redevelopment of this massive site including to convert it into a retirement village and riverside walkways and other facilities.
Added Cllr Hassan: “Under the newly released Strategic Growth Plan for Derry and Strabane 2017-2032 it predicts the over 65+ population in the district will increase by 66% in that period.
“We are actively working to ensure our elderly are catered for in terms of suitable accommodation and care.
“We need to be proactive in turning this site into something positive not only for the people of Culmore but indeed the wider city.”