Outraged parents and scared students have been demanding answers as the third mass poisoning has been reported at a school in the southern Mexican state of Chiapas, according to local authorities.
At least 57 pupils from the Juana de Asbaje secondary school in Bochil municipality, were taken to hospital on 7 October showing symptoms of having been poisoned by an unidentified substance.
According to reports, the symptoms ranged from nausea and drowsiness to abdominal pain and fainting. Parents were cited as speculating that the students had ingested contaminated food or water at the cafeteria during a Friday school event.
Footage has been circulated on social media showing the chaotic scene at the local hospital as teenagers in school uniform were being rushed to receive medical assistance.
After claims by some parents that the pupils had unknowingly ingested cocaine, the state prosecutor's office said on Saturday that, after toxicology tests, 15 samples came back "negative for prohibited substances, or drugs of abuse". However, the office added that further testing would be conducted.
Earlier, a video on social media reportedly showed one man saying his daughter had been poisoned and tested positive for cocaine at a private laboratory, along with other pupils.
The authorities have not made any official statements regarding the cause of the mass poisoning. The prosecutor's office said it was investigating the incident. However, alarm has been growing as local media reported that since 23 September there have been two other cases of poisoning affecting dozens of students in the city of Tapachula, in the far south-east of the state of Chiapas.