If you are on a Chromebook that supports Android apps, check the Google Play Store. The mobile version of Rocket League (Sideswipe) is a 2D/2.5D version of the game officially made by Psyonix. It is far superior to browser games and is usually allowed on school devices if the Play Store is enabled.
If the primary URL is blocked at your location, do not give up. Here is the triage process:
By searching "Rocket League 2D Unblocked Games 66," you are specifically looking for the 2D version hosted on a domain that your school’s network will not immediately kill.