That spinal fluid thing is not true, no school in their right mind would pay upwards of $2000 USD (and you'd have to sign a waiver form for it, for risk of infection, nerve damage, ect) and spinal fluid wouldn't tell them shit- THC broken down does not break the blood/brain barrier anyway, it stays in your blood until it goes into your fat tissue.

What are they threatening if you come up positive?

Shampoos will not work, much like detox products- they test the cortex of your hair, not the cuticle, and shampoos only penetrate the cuticle, so don't pay 40 bucks for that bullshit.