On the PC, I used a free TightVNC server for screeencasting. Students connected to it from the class PCs with a free viewer app and saw my demonstrations in a window, but could not do anything on my desktop.
I am able to connect from a PC to macOS's built-in VNC server if I supply a password (VNC viewers may control screen with password) in System preferences. However in this case, after connecting and entering this password, I am presented with the normal Mac login screen, after which I am controlling my Mac. I need a view-only connection.
If the password box is unchecked, TightVNC Viewer fails to connect, saying No security types supported. Server sent security types, but we do not support any of theirs.
TigerVNC does not have a server for macOS, only viewer.
RealVNC is paid and seems to use the cloud to connect clients to their VNC server, which I'd rather do without, after all, we're on the same LAN in class.
VineServer (osxvnc) seems to be abandoned and anyway crashes immediately upon start.
Best Answer
Apple’s screen sharing is designed around security, so having multiple views in would be my last resort.
How about something like Skype for Business / MS Teams / Google Hangouts. None are truly free - you are selling access and attention to Google if you use their option and Microsoft charges for O365 to host the meeting tools. There are dozens of other collaboration tools like Slack that also enable sharing as well as the vendors like WebEx, Zoom, GoToMeeting, TeamViewer and handfuls of other startup / smaller collaboration tools.