IANAL, but "concealing" case law implies that the knife is not apparent to a "reasonable person" and is readily accessible. Since retrieving a FB from a backpack (California considers all fixed blades as dirks or daggers) requires several steps, then it is not readily accessible even through the knife is not apparent.

There was a case a while back where somebody with a Swiss Card in their wallet was tried for concealing a dirk or dagger. The judge ruled that since it took several steps (pull out wallet, open up wallet, retrieve Swiss Card, pull out knife) that it was not readily accessible, therefore not illegal. Had he been carrying the Swiss Card IWB (not apparent and readily accessible), it would have been another story.

There are statutes for brandishing, threatening, or other misbehavior with a "deadly weapon" (section 417). A ‘deadly weapon’ is ‘any object, instrument, or weapon which is used in such a manner as to be capable of producing and likely to produce, death or great bodily injury. So cracking them over the head with your skateboard is the same as pulling a Condor Combat Machete on them.