The boy said Kara Loofborough exposed her breasts and vagina as they talked on the internet.
And, while at school, she showed him footage of her performing oral sex on another male, he told police.
Loofborough, 26, also reportedly sent him a link to a porn site. She admitted to several incidents involving the student, said police.
They included showing her genitalia to him during a video chat and masturbating while he watched.
She said she knew how young he was, police added.
But she also owned up to having him kiss and touch her breasts.
Loofborough was arrested after the the boy’s father got worried about the after-school contact his son was having with his science teacher at Fees College Preparatory Middle School, Tempe, Arizona.
A letter sent by education officials to parents last Friday said that Loofborough had been a seventh-grade teacher in the district since August last year.
But she had not been at the Middle School since March 15.
She resigned for “personal reasons” on March 30, to take effect from May 1.
“At the time she was hired, she had no criminal history reported based on the District’s fingerprint,” said the letter.
“Record checks with the Arizona Department of Public Safety and her background check by the District revealed no adverse findings.”
Loofborough has been charged with one count each of indecent exposure, luring a minor for sexual exploitation and sending obscene material to a minor. She remained in custody after a judge set her bond at $100,000.