Not sure if this would make a difference as well:
https://powerbi.microsoft.com/en-us/blog/announcing-the-new-viewer-role-for-power-bi-workspaces/
Looks like if the dashboard is in a pro environment, which you would need one licence for, you can then have as many free viewer accounts, is that the case? If so schools would only need to pay for one pro licence to publish the dashboard and then they can invite as many read-only viewers to it as they want without them having permission to change anything or needing to have lots of pro licences.