Anything that's forbidding is a little menacing, daunting, or even frightening. It's hard enough starting at a new school, much less one in a huge, grim, forbidding building.
Things that loom high above you, dark and cold, are often described with this adjective. Think of the forbidding trees lining a spooky path at night, or your stern, forbidding math teacher, silently frowning down at the class with his arms crossed. Forbidding comes from forbid, "refuse to allow," and its Old English root meaning "command against."