"Unblocked" games are simply browser games hosted on domains that school firewalls haven't flagged yet. They aren't "hacked" versions; they are just hosted on sites that look innocent to the firewall.
When downloads are completely restricted, browser-based "unblocked" sites are a popular alternative. These sites host the game in an HTML5 format that runs entirely within a tab. Unblocked Games FWD - Subway Surfers - Google Subway Surfers Download For School Chromebook