What users see on blocked websites depends on whether your school uses Linewize Connect and how it’s configured. If you don’t use Linewize Connect, users will see the default “Page can’t be displayed” message for their browser.
If your school deploys Linewize Connect to users' devices, you can turn on a Block Page to show users that they’ve been blocked. Block pages look different depending on the device type and the filtering mode your school uses.