Thank you DreymaR. My ideal solution would be to have a portable (windows) version of xtrlock (which is a light Linux locker). Via PKL I hoped to get a similar solution. And yes I do need PKL, because I need admin rights for AHK which I don't have.

(off topic remark: why don't corporate IT departments COOPERATE with "power users" like me? )

Anyway, I'll look into the source code of AHK. See if I can cross compile it (that is: compile a windows program on my Linux machine; compiling is no problem but I have never compiled for or on Windows machines - I switched to Linux long ago because I found Windows too hard and Linux much more logical ). Perhaps easier is to dive into the hell hole of freeware (I'm used to the clean Linux repository world) and find a free portable screen locker, AND combine that with (PKL) disabled alt/ctrl/del keys, to prevent students from going to the task manager........

Thanks again DreymaR.

I was not clear on the compiling part, with cross compile I meant that I use my linux machine to compile a program for a windows machine, I did not mean that I adapt the program for use on linux machines).  But again, I'll look into PKL code, Thanks again, DreymaR !

It works!  This morning I browsed on the Autohotkey sites, read some man pages, and wrote an unelegant, but working, script. All keys and all mouse buttons get ::return  (meaning that they do not give input to the computer) except for a few modifiers. A modifier combination gets ::ExitApp  (meaning they end the script's running, meaning that afterwards the keyboard & mouse key function normally again.

Then I borrowed my wife's windows-laptop, downloaded Autohotkey, tested the script and then compiled it, which can now be done within Autohotkey itself and has an idiot-proof menu. Even I can do it! ;-)  By the way, I think that it is not really compiling (=translating higher level code into lower level code) but I think it is just "making portable". Anyway, you end up with a windows .exe file that does not have to be installed and can be run as a user (!).

How does "my" little program work?  
- Copy the .exe file on your PC  (let's say on drive D:\). Click it, and the script will run.

What does it do?
It blocks all keyboard and mouse keys. So, you can still see the screen, you can move the mouse and even see the pointer, but if you click or if you push a key, nothing happens. (This resembles the (linux) screenlocker xtrlock).

What is this bad or good for?
This is a bad solution if you want to protect " sensitive" information from nosey people. But for my use it is very good: as a teacher I often show students things on a big screen (assignments, Youtubes, Powerpoints etc.) and they may (they must!) see that. But I don't want them to look up other things on that computer, or to access my files and so on. Now I can show them an assignment and walk away for 5 minutes without having to guard the PC all the time.

How does the script stop?
- By entering the secret modifier combination

What does ctrl-alt-del do?
- This Windows key combination behaves remarkable. Despite me having "muted" the ctrl, alt and del keys, the combo still works, meaning tghat it overrides the AHK script. Next, you can even start the task manager. My fear was that a clever student could then see the script running, kill it, and have access to the machine, but the crazy thing is that yes, I can open the taskmanager, but then the mousekeys & keyboard keys stop working again, so I cannot kill anything. Couldnt even get to the screen that shows all running processes.....

To do:
- test on school computer
- important: see how I can block the input from the Smart Board (!) I forgot this, but we have smart boards, which are like tablet screens. If I click on a button on that screen, it is like I pressed it with a mouse. The smartboard is actually some sort of mouse, I hope that it is blocked (if not I'll have to dive in again).

Well, this wasn't to hard :-)  :-)

Yep, it works great. The smartboard is also blocked, it looks like the input of the board is interpretated as mouse clicks, as I hoped. I may as you say include this script into PKL.

What I would also like is some sort of visual clue that the script is running. This also deters people from trying to do stuff on the computer. I was inspired by xtrlock, the minimalistic linux locker, which does the same: it lets people see the screen, but blocks all user input. In xtrlock you can move the mouse, but you cannot scroll, click or use the keyboard. The visual clue xtrlock gives, is that the mouse pointer is replace by a blue lock. If you move the mouse, the blue lock moves. I will see if I can imitate that with AHK.