Because FNF is a browser-based game (the original and many mods run on Newgrounds or Itch.io), those domains are often on the block list.
Some unblocked game sites (like CrazyGames, Y8, or dedicated FNF unblocked sites) host web-optimized versions. These can run in the Chrome browser without downloading anything. Search terms like “FNF unblocked 76” or “FNF unblocked 911” refer to specific mirror sites. fnf unblocked for school chromebook