Dingwall Academy was forced to send out the letters after several pupils were believed to have consumed the substance.
The letter warned the liquid was “highly toxic and can be fatal”.
Highland Council confirmed letters had gone out to pupils’ parents and carers on Friday.
A wider warning has been issued by NHS Highland about the potential risks
There were reports of one pupil from the school being taken to hospital paralysed after drinking the liquid.
Symptoms of an overdose can vary from burning of the mouth, nausea, vomiting, headaches, dizziness, to cardiac arrest.
A spokeswoman told BBC Scotland: “We have had to deal with a small number of incidents over the last couple of days where pupils have become seriously ill in school.
“We believe that the illnesses may have been caused by individuals drinking vaping fluid, which is highly dangerous.
“We have spoken to pupils at assemblies and have sought further advice from the police and NHS Highland.”
Dr Jenny Wares, consultant in health protection at NHS Highland, said: “When ingested, the contents of the fluid are highly toxic and can be fatal.
“The main risk of toxicity is from the nicotine content although there is a risk of toxicity from other ingredients within the fluid.”