Video stream quality is up to whatever the software package that is being used for the school streaming sessions.
You may or may not have any choice in this depending on whatever software package the school has chosen..
On popular interactive multimedia streaming software that is used by businesses call Zoom, does have controls that allow you to selct video quality..
Found that
HEREYou can do a Google search with the name of the conference software that your school is using to see if there is any controls you can set.
For my search I used "limiting video quality on zoom" as my search, just sub in the name of the software you have to use instead of zoom and let the search fly.
Otherwise perhaps contact your schools staff to see if they can assist, let them know that you have 3 children streaming at the same time AND you are on a limited bandwidth Internet provider with data limits/caps..