Here's the info on the Texas Helmet Exemption Sticker.
http://www.txdps.state.tx.us/msb/
Frequently Asked Questions.
Do passengers have to meet the same age/insurance/training requirements as the operator?
It is presumed that operators and passengers on a motorcycle displaying a helmet exempt sticker on the license plate or plate holder meet the age/insurance/training requirements.
No helmet exempt sticker on the motorcycle? If the motorcycle owner has the state-required minimum insurance, passengers, providing they are at least 21 years of age, can ride on a motorcycle without a helmet.
Is the helmet exemption sticker required before a motorcyclist can ride without a helmet?
No. The sticker indicates that the motorcyclist meets the requirement for riding without a helmet, and law enforcement officers will presume that they do. Without the sticker motorcyclists may be requested to show police officers their proof of course completion or insurance coverage.