Hello there leannefromhighland,
We understand you're seeing some issues while trying to screen share on certain student's computers. To get started, do these computers have remote management enabled with control through the school, or are they just owned and controlled by the students/parents?
The reason we ask is unless they are secured with Remote Management, there would not be any way for you to remotely force them to enable certain settings, so you may need to reach out to the parents directly for additional help. Otherwise, you'd want to ensure they haven't disabled any access to screen sharing as shown here: Turn Mac screen sharing on or off
Next, if you have remote management enabled on their computers, then you or your IT Administrator may need to review their settings or check for any third-party apps or software that could restrict or filter what you may see during screen sharing.
We hope this helps!
Take care.