My most up-to-date NFHS Rules Book is 2013 but I don't think much has changed.
Rule 5, Section 10, Article 10 states:
"In relay races where acceleration zones are not permitted, each outgoing competitor, while waiting for the baton, shall take a position entirely within the passing zone and must complete the exchange while the baton is within the 20-meter exchange zone."
Boldface added by me
From the 2013 NFHS Case Book:
SITUATION: In a relay race in which the acceleration zone is not permitted, the outgoing relay runner A2 takes a position just inside the beginning of the exchange zone. As incoming runner A1 approaches, A2 begins to accelerate by taking a step backward with the right foot outside the limit of the exchange zone. RULING: Violation.
The 2017-2018 NCAA Rules Book (not applicable to high school obviously) states it a little better in my opinion:
Rule 5, Section 8, Article 4 states:
"... outgoing runners shall start their running after establishing a position entirely within the 20-meter passing zone, not touching the ground outside the zone."