Twelve of the 13 school websites that were hacked on Wednesday afternoon were still down as of 9am on Thursday.

They are: Bukit Timah Primary, Canberra Primary, Compassvale Primary, Greendale Primary, South View Primary, West View Primary, Woodlands Ring Primary, Henderson Secondary, Kent Ridge Secondary, Maris Stella High, St Gabriel's Secondary, and St Andrew's Junior College.

Only the Raffles Girls' School (Secondary) site was operational.

The websites were hosted on the same server. They were hacked by a person or group that signed off as "Jack Riderr".

The Education Ministry said in a statement on Wednesday night that the 13 websites were reported defaced between 3.30pm and 5pm on Wednesday. It added that it had referred the matter to the police for investigation and are working to restore the affected sites.