Zoe Gregory, 26, sent a message claiming an explosive device had been left at Ormiston Victory Academy in Costessey, Norfolk, on 9 February.
Gregory appeared before Norwich magistrates and was committed to Norwich Crown Court for sentencing.
The pupil's home was raided and she was questioned over the threat by police.
Gregory, of Blackhill Wood Lane, Costessey, has been dismissed from her role, a school spokeswoman said.
Students Holly Littlefield, 16, and Vicky Francis, 15, were arrested and held in police custody for 10 hours after the scare, the court heard.
But it later emerged Gregory had accessed Miss Littlefield's email account without her knowledge.
Mother-of-two Gregory pleaded guilty at Norwich Magistrates' Court to carrying out a bomb hoax and unauthorised access of an email account.
Prosecutor Lisa Britton said both girls had been left distressed by the ordeal and had been teased at school.
Fellow pupils called them "terrorists" and dubbed Ms Littlefield "Holly Bin Laden", the court heard.
Chris Brown, mitigating, said Gregory had been "plagued by post-natal depression" and acted out of "sheer desperation".
A spokeswoman for Victory Academy said: "We have been co-operating fully with the police.
"Appropriate processes have been followed throughout.
"We continue to do everything we can to support the children involved."
A teaching assistant has admitted using a pupil's email account to make a bomb threat against the school where she worked.