Tanikka Queen, 22, was a long-term substitute at Hyde Park Middle School, where she taught English and geography.
Police said the evidence against Queen includes thousands of text messages, up to 100 phone calls and photos exchanged with the 15-year-old boy.
Queen, who was taken into custody on Tuesday, hasn't been on campus since Feb. 26. She's facing charges of kidnapping and sexual assault.
Rumors about the relationship, which allegedly started after Valentine's Day, were rampant on campus. Those rumors eventually reached an unidentified parent, who contacted school police.
Police launched an investigation, and Queen was suspended.
"Any type of rumor, allegation that a teacher or employee of the district is having any relationship, we investigate that. It's a priority for us to make sure students have a safe learning environment," said Sgt. Mitch Maciszak with the Clark County School District Police Department.
According to text messages she sent, Queen knew she was risking her job just speaking to the student outside of the classroom, something she learned in teacher training.
Police said Queen confessed to having sex with the teen one time.
"We hope we don't have this again; however, people make conscious decisions every day between right and wrong," Maciszak said.
Substitute teachers are subject to the same background check as regular teachers and staff. Queen's came back clean.
Police on Thursday said they do not believe any other students were involved with Queen. They urged parents to keep tabs on who their kids are talking to.
Queen is scheduled to make her first court appearance on Friday at 7:45 a.m.
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