True.
But it would effectively stop innocents sticking a time bomb (that they found laying around) into a computer.
Right. Except instead of that they stick in a software "bomb" in the form of a virus/ransomware infested SD card or USB stick.
What is the difference? The zapper at least manifests itself in a fairly obvious manner right away, so whoever decides to vandalize a computer in the lab is going to be quickly caught. Most university labs that I have been to require some sort of access control - students have a keycard or there is at least someone on duty keeping an eye on the lab. Often both. So the set of potential suspects is fairly small. Also university students are usually not this stupid to entertain themselves in this way because they know they would likely get expelled and, very likely, prosecuted.
A ransomware infected stick is a way worse problem, IMO - that could put the entire lab (or even school if they are sufficiently clueless) out of commission and good luck catching the culprit, because the infection takes some time to take effect. And nobody is up in arms about this.
Let's not try to engineer a BS solution for a non-problem, OK? E.g. those optoisolators are next to useless for mass storage devices, because they usually support only USB 1.1. Nobody wants to wait for hours to copy the homework. Network access can be also fairly limited - many places run restrictive firewalls and various censorware (often mandated by law for public institutions), so accessing even something like Google Drive or Dropbox can be a challenge. I am also not sure I would want to log into my personal account from a public computer anyway.
So this kind of technical "solution" that ruins the USB ports for everyone for the off chance that some moron decides "to have fun" is probably worse than the problem it is trying to address.