If his account wasn't the controlling account, and the school really did lock themselves out, they started the problem. If he used rng for a good strong master organizational email password, and it got wiped as the laptop got returned, he may not have it to return. (one wonders about the state of the school's backups...) As an employee you can't just assume the school is going to go retard on you and require you to provide copies of stuff they ought to already have. To the school's credit, he ought not to have wiped the computer before returning it, that's his bad.
When I last changed jobs, it was well known that I had copies of work-related data on personal drives, as I mirrored them to several around the shop for everyone to use the tools and data on. I was asked to delete that data on my personal drives when I left, which I did. I found out months later that the GM went on a wiping spree, intent on nuking ALL the service drives. (bright lad, that one) I was asked later by the SM if I had that data. nope. The SM finally found one last service drive in an old service machine that had been replaced and mothballed, saving enormous headaches. If they'd have lost that data for good, tough. NOT my problem.
It does sound like Williams isn't going out of his way to be cooperative, but it also sounds like the school is expecting more than they are entitled to in the way of cooperation. Will need to get more details on both sides. Even if he "violated policy" while he was working there, that'll be tough to find any legal liability over. You fired him, that's what you do when they violate policy. That doesn't also mean you're allowed to fine, sue, or break his knuckles after you've parted ways.