Firewalls rely on "SSL Inspection." To inspect traffic to sites.google.com , the firewall would have to break Google’s HSTS (HTTP Strict Transport Security). Modern browsers will outright refuse the connection, throwing a security error. Most Network Admins whitelist Google entirely.
We must address the elephant in the room. Creating a portal to bypass your school’s Wi-Fi to play Fortnite or access social media is generally a violation of the school’s Acceptable Use Policy (AUP). Consequences can range from a warning to network suspension. nebula proxy google sites
The method is arguably the most resilient way to bypass network restrictions available to the average user today. Firewalls rely on "SSL Inspection