Hi,
Based on your description, you have a Chromebook owned by a school, which means that the school administrator may have restricted access to gaming websites and apps. Unfortunately, you need to ask permission to your school IT administrator for permission to allow access to gaming websites or apps on the Chromebook.
I suggest using a personal device to access Roblox since it does not have the same restriction policy.
It may not be the outcome or solution you were hoping for, but I hope this helps address your concern.
Stay Safe!