Beggs, 17, is transitioning from girl to boy, and the governing body for Texas school athletics has required him to compete by his birth gender, which is female.
The wrestler at Trinity High School in the Dallas suburb of Euless had a 52-0 record ahead of the tournament and is the favourite to win his 110-pound weight class in the girl's championship, which ends on Saturday.
Beggs' family has sought to have him wrestle as a boy, and some of his opponents have said he has an unfair advantage among girls from the testosterone he is taking as a part of his transition.
The University Interscholastic League, which governs school sports in Texas, said that the state's education code allows the use of a banned drug such as steroids if it "is prescribed by a medical practitioner for a valid medical purpose."
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Beggs and his coaches have declined media requests to speak ahead of the tournament.
About a week ago, Beggs won a regional championship after a female wrestler from a Dallas-area high school forfeited the final.
The parent of another girl who wrestles for the same Dallas-area high school had filed a lawsuit trying to block Beggs, saying his use of testosterone increases strength, which could pose harm to opponents.
Nancy Beggs, Mack Beggs' grandmother and guardian, told the Dallas Morning News after the forfeit in the regional championship match: "Today was not about their students winning. Today was about bias, hatred and ignorance."
According to transathlete.com, which provides information for transgender athletes, Texas is one of seven U.S. states with policies it sees as discriminatory against transgender athletes.
Lou Weaver, who runs transgender programs for the LGBT rights group Equality Texas, said Beggs is abiding by current state rules, which need to be updated, "so that guys like Mack can wrestle with their peers, which would be on the boys' team.
"He is passionate about wrestling. He is living his dream and he is wresting for his high school," Weaver said.
(Reporting by Jon Herskovitz; Editing by Cynthia Osterman)
