Power cables worth tens of thousands of pounds were stolen from temporary classroom buildings, a school has said.
Parents were told that Sir Frederick Gibberd College (SFG) in Harlow, Essex, could not open for the day because of an "unforeseen power issue".
The school said it would be closed on Tuesday too and "potentially for the coming few days".
Teaching has been repeatedly disrupted at the school since September over building safety concerns.
Parents were told about the sudden closure via email at 06:55 GMT on Monday.
Helena Mills, chief executive of the Burnt Mill Academy Trust which runs SFG, wrote to families saying a "well-planned, sophisticated theft" took place over the weekend.
Ms Mills added: "The SFG site was broken into and tens of thousands of pounds worth of power cables were stolen. As such, there is no power on-site."
The majority of pupils would be taught via online learning and SFG also told parents via email it was "committed to minimising disruption to your child's education".
Former Harlow Council Labour leader Mark Ingall, who has two children at the school, told the BBC the closure was the "final straw" and said he was "exploring finding an alternative school".
Fellow parent Sara Cordwell, whose daughter is in Year 10, said: "It's becoming rather stressful."
FSG closed its main building and sports hall in August over concerns that its "modular mode of construction" would not withstand extreme events such as high winds or a big snowfall.
Most students returned on 14 September, but the school was closed once again for a fortnight from 30 October because of improvement works to its temporary classrooms.
The FSG site first opened to pupils in 2021.
The BBC contacted Essex Police about the alleged theft.