Logging in clients from the master computer isn’t working

[Anne K]

I can't log in to any other computer in the same room from the teacher's computer.
I can restart it or shut it down. And if I've manually logged into a student's computer, I can also send files to it, log out of it, etc., from the teacher's computer. But I can't log in to one or more computers. I can do this via “Remote Control,” one computer at a time. But not via the “Log In” button. Not a single computer, not multiple computers I’ve selected, and not “All” from the menu at the top.
We actually use this feature very often (and I thought it was one of the main features). When second- or third-graders are supposed to learn on the computer, they can’t log in on their own yet. They’re supposed to get familiar with the keyboard first. But there are 26 kids sitting there. I simply can’t manage to log them all in manually, one by one, during a single class period. In the class before or after, there are sixth graders who log in using their own login credentials. So I also can’t log all the computers in manually first thing in the morning for the rest of the day.
As a result, we unfortunately either have to cancel the class entirely or only half the class can work.

Please help me as soon as possible. What could be causing this problem? Everything was still working fine just a short while ago. We’ve been using this system for several years now.

Thank you very much for your help, and best regards.