SHERBROOKE, Que. - It was an end-of-year school activity featuring a hypnotism show and it didn't go quite as planned.

A Quebec all-girls high school said the activity went awry as numerous students had problems after the show — including one girl who remained stuck in a trance for four hours.

The incident occurred last week at a private school, College du Sacre-Coeur in Sherbrooke, during a lunchtime end-of-year event.

Thirteen students reported having headaches or nausea. At least five other appeared to experience more serious trouble after the show, given by a 20-year-old hypnotist. Some were in a daze with their eyes open. At least two were reportedly conked out on a table.

The school said one girl remained in a trance for four hours — which principal Daniel Leveille, in a statement Friday, described as a "deep sleep."

School administrators had to call in the hypnotist's mentor, who came to help deal with the problem. The mentor and trainer, Richard Whitbread, brought the teens back to regular consciousness.

Administrators said they learned after the fact that hypnosis isn't recommended for people under the age of 14 because people that young are particularly susceptible to it. The school said through a spokesman Friday that it had no idea such a show could have those side effects.

Leveille said the administration plans to hold a news conference next Tuesday to provide updates. In the meantime, they're keeping tabs on students affected by the show.

"This incident raises questions about our practices, which we wish to discuss to ensure similar events don't occur again," Leveille said in a statement.

"We also wish to gather all relevant information before talking to the media about this unfortunate event which, fortunately, did not leave any consequences."

Some 450 students between the ages of 13 and 17 attend the Eastern Townships school, about 150 kilometres east of Montreal.

The young hypnotist, Maxime Nadeau, was quoted in a news report saying he did not panic because he knew the students were never in actual danger.