3 minutes
I tested this on three different school-issued Chromebooks across two networks (one using GoGuardian, one using Cisco Umbrella). Here are the results:
Symbaloo is a visual bookmarking tool used by teachers to create "webmixes" of approved resources (think: Britannica, Khan Academy, Google Docs). The number "76" refers to a specific, viral user-generated webmix. Some anonymous hero in 2019 figured out that if you embedded an iframe of an unblocked games site inside a Symbaloo tile, the filter would see [Symbaloo.com] and let it pass, rather than seeing [UnblockedGames77.com].
