Start a teensy sketch after USB handshake has completed or upon a button has been pressed
Having been in Shannon's position, I can understand what he's trying to do.  Doing this by remote won't help.  You have to physically go to each machine, turn it on, and check not just that it boots, but that the drives are OK, that the keyboard and mouse work, that the monitor is OK, etc.  Last but not least is of course seeing that it is where it should be and updating the database if the PC has been moved (or putting it back where it belongs.) 

This is as much a physical inventory as anything else.  Since it is summer, I would also expect the PCs to be shut off to save power and reduce the school's electricity bill.

Horta's suggestion of using VNC is therefore out of the question - and VNC would be a bad choice, anyway.  In a Windows System you would do better to use RDP (which is already installed compared to having to install and configure VNC 500 times.)  Under Linux, you would have a good many options but still, Shannon has to go and physically check the machines.

I would also hope that the PCs in the school cannot be reached for VNC (or even RDP) over the internet.

I would have preferred to make this a comment to Horta's comment, but I don't have enough reputation for that.