The pages before, the girls are all surprised that Monica was able to drink it all.
"She drank it?"
"Are you serious? But it's so bitter."
"No way, I thought for sure she'd just choke on it."
So they definitely put something in there that was not intended to be consumed. Best case, they just got the dosage wrong.
I do not expect the best case. That would involve schoolgirls knowing anything about dosages.