Safari 15.6.1 does not start Java applets and downloaded Java applets will not start

My son urgently needs to run Java applets for his maths and physics courses at school.

1. I installed the latest version of Java from Oracle.

2. Under Macintosh HD > Library > Internet Plug-ins JavaAppletPlugin.plugin is installed.

3. Upon restarting Safari, no Java applets load from the websites provided.

4. The downloaded .jar files, provided for offline use, also won't start after a double-click.

5. I tried the same on my own MacBook and on that of my wife's, to no avail.

What can I do to run Java applets in Safari or after a download?

Thanks!