I have looked in Help but there is no mention of de-activation and I did consult on Chat but they said go to the forums.
Have solved the problem.
There is no activation process with Lightroom. Once you have received your serial number for your student/teacher edition, you treat Lightroom the same as any other version. You install Lightroom on the other computer and use that serial number that begins with the digits 1160 to activate it. Don't try to download the creative cloud version and install it because you cannot activate it with a serial number.
Worked just as you said.
All the best,