Iria Suarez-Gonzalez did not have to disclose her criminal past because the conviction was regarded as spent by Spanish authorities.
Aged 16, she was jailed after she and a friend killed teenager Clara Garcia in 2000.
The pair reportedly drove Ms Garcia to a field and stabbed her 32 times, according to previous media coverage on the case.
After her release in 2006, she moved to the UK and applied for the role at West Oxford Community Primary School, starting work in September 2016, but did not need to disclose her murder conviction.
Staff, parents and teachers are said to have been unaware she was a killer.
She left the school in July 2017 and her criminal past came to light when it was reported to Crimestoppers by an anonymous caller.
Police told Oxfordshire County Council in October 2017.
Earlier this year, Suarez-Gonzalez appeared at Oxford Crown Court charged with fraud by false representation for allegedly not disclosing the murder as a previous conviction in her job application to the school.
But the case has since been dropped by the Crown Prosecution Service (CPS), reportedly due to a lack of evidence.
Reporting restrictions on the case were lifted by crown court judge Nigel Daly on Friday after the Oxford Mail newspaper appealed against the order.
A CPS spokeswoman said: "The CPS keeps all cases under continuous review so we can consider any new information that comes to light.
"In this instance, it emerged Ms Suarez-Gonzalez's conviction was spent which resulted in the charges being dropped."
This information was not made available until the trial preparation hearing, at which point a different view was taken on the case, according to the CPS.
If judicial sanctions are imposed on a child and 10 years have passed after turning 18 - provided sanctions have been enforced or expired - it is at the Spanish ministry of justice's discretion to expunge these records, the CPS said.
Suarez-Gonzalez had nothing to declare on her application because the convictions were removed in line with the Spanish ministry of justice's protocol and it provided certification she had no criminal record in order for her to fill in the application, according to the body.
A spokesman for Oxfordshire County Council said: "The school had followed safer recritment processes in terms of the recruitment of this individual.
"Information had been provided anonymously through Crimestoppers.
"Police alerted us in October 2017, by which time she had already left the school.
"There is a clear recruitment process in place for schools to follow in line with safer recruitment procedures.
"The person in question was a teaching assistant in foundation stage from September 1 2016. She left the school in July 2017.
"We know this has been a very difficult time for the school and parents.
"The school are in liaison with the council to continue to support students through the curriculum around feeling safe and knowing they have people that they can talk to in school if they need to.
"We would stress this charge was not in any way related to any children at the school and only came to light after the staff member had left the school."
The school has been contacted for comment.