Two teenage girls have been arrested on suspicion of plotting to kill their teacher, with one of the girls allegedly carrying a knife in her schoolbag.
The arrests come days after the murder of Ann Maguire, 61, who was knifed to death in a Leeds classroom.
Officers were called to Cwmcarn High School, near Newport near Newport in south Wales by a teacher at nearly midday on Thursday. Police said no physical confrontation had taken place.
A 15-year-old girl from Newbridge and a 14-year-old girl from Risca were arrested and later bailed pending further inquiries.
The 15-year-old was arrested on suspicion of threats to kill, possession of a "bladed article" on school premises and conspiracy to commit murder. A kitchen knife is believed to have been found in her bag.
Her 14-year-old friend was arrested on suspicion of conspiracy to commit murder.
The alleged target, Alison Cray, 46, who has taught maths at the school for 10 years, said she had returned to work as normal.
"I know very little about it. It all happened elsewhere and I wasn't directly involved at all.
"I've been very lucky that I'm at Cwmcarn and that they have handled it so well. I've been working as normal and I'm fine," she told the BBC.
In a statement on the Cwmcarn school's website, head teacher Jacqui Peplinski said that staff and pupils were safe.
"You may be aware that concerns were raised about a pupil in the school yesterday.
"Please be assured that the concerns were dealt with swiftly. There was no risk to any staff or learner and the relevant support services were contacted as a precautionary measure.
"Cwmcarn High is a caring and safe environment and we are committed to everyone's safety. Please do not hesitate to contact us if you have any concerns," she said.
One line of police inquiry is that the girls may have been motivated by the murder of Maguire at Corpus Christi Catholic College in Leeds earlier in the week.
A 15-year-old boy has been charged with her murder, and was remanded in custody at Leeds Crown Court on Friday.