Is headwear restricted by NFHS?

[Theisey]

If seen outside the helmet, it becomes or falls under the adornment rule.
Otherwise, it's not an issue.

That being said, I have heard of some places that make the wearing of this device illegal because of "gang" association. That's a different issue and I have no complaint about it.

[HLin NC]

for two reasons- 1) potential gang affiliation and 2) the tying of the bandana creates a knot which creates a protrusion into the helmet, thus a safety issue to the player. If seen, it was considered to be uniform adornment.

The new, knit, skullies by Nike, UA , et al are deemed ok.

[KSRef]

Illegal in KS. Safety issue. Helmets are designed to fit snuggly on the head. Anything that interferes with that fit could cause injury. Knits, scullies nothing can be worn under the helmet.