M&S has also been targeted with ransomware - malicious software which locks an owner out of their computer or network and scrambles their data.
The criminals then demand a fee to unlock it. Sir Dan says it was a demand he resisted.
Instead, the school group approached a firm of cyber specialists who employed a hostage negotiator. That individual then took on the role of an inexperienced school bursar - an administrator - who pretended to not know what was going on.
They took up negotiations with the hackers, with the purpose of delaying them for as long as possible so the school group could rebuild its systems.
"The Russians had stolen data from us - they didn't tell us what - and they threatened to put this stuff up on the dark web and cause us great embarrassment, and secondly they would lock down our systems."
Sir Dan said it took the Harris Federation three months to get everything working again, at the cost of £750,000. Among the work was 30,000 devices that needed to be "cleaned" following the hack.
Was there ever a question of giving the criminals what they wanted? Never, said the school group boss.
"The money we have is for disadvantaged young people, and secondly had we paid we would have opened the door for other school groups to be attacked."
