Proving criminal negligence, in general, can be a challenging task for prosecutors. And in the case of an Ontario teacher facing that charge in connection with the drowning death of a student, legal experts say the Crown will be facing an unusual set of circumstances.

"Criminal negligence causing death is not exactly the easiest charge in the world for the Crown prosecutors to prove," said Daniel Lerner, a Toronto-based defence lawyer who was a former Ontario Crown prosecutor.

Lerner said he hasn't come across any similar cases before, where a teacher has been charged for something that occurred on a school trip.

"It is somewhat unprecedented. But the unique factor here is accidents happen; accidents don't lead to criminal charges."

The OPP announced Thursday that Nicholas Mills, 54, had been charged with criminal negligence causing death, one year after 15-year-old Jeremiah Perry drowned during a week-long field trip to Ontario's Algonquin Provincial Park.

Jeremiah was three days into the outdoor-education and canoe excursion last July when he went for an evening swim with classmates and disappeared under the water. His body was found by a police rescue unit the next day.

His family has said the teen did not know how to swim.

Jeremiah Perry, 15, drowned while on a school canoeing trip to Ontario's Algonquin Provincial Park in July 2017. (Submitted by the Perry family)

A third-party review found Jeremiah had not passed the Toronto District School Board's mandatory swim test and was not wearing a life-jacket. Students were required to pass the test before going on such trips.

The school board also reported about half of the 33 students on the trip had failed the swim test — which involved water safety, lap swimming and underwater endurance evaluations — but were allowed to go anyway.

According to the OPP, Mills had organized the field trip and was the team leader assigned to Jeremiah's group that day.

'Marked departure'

Under the Criminal Code, someone is considered guilty of criminal negligence if "in doing anything, or in omitting to do anything that it is his duty to do, shows wanton or reckless disregard for the lives or safety of other persons."

"[The Crown] would have to prove that either the teacher committed a specific act, or omitted to do something which he had a legal duty to do," said Seth Weinstein, a Toronto-based criminal lawyer. "So they have to prove one of those two requirements — not both."

What's key, according to legal experts, is whether the actions of Mills were a "marked departure" from the actions a reasonable person would take.

"We're looking for something well beyond what a reasonable person would have done, and something very bad happened as a result," Lerner said.

One factor in this case, he said, is the issue of not yet knowing exactly what the school policy was regarding the canoe trip: Was that policy followed, to what degree was that policy followed, and in particular, how did it relate to the failed swim tests.

Jeremiah was three days into a week-long canoe excursion last July when he went for an evening swim and disappeared under the water. (Grant Linton/CBC)

A number of specific details surrounding the tests and Jeremiah's death have not been shared publicly, and the TDSB said it had put its own internal investigation on hold at the direction of the OPP.

"Going into that canoe trip, there may or may not have been these red flags. And I think that's what creates the unique circumstance here — and you don't hear about that that often," he said.

The case could come down to such questions as what exactly does it mean to have failed the swim test, Lerner said.

Failing a swim test could mean a person was only able to do six laps instead of the required seven, and the person was allowed to go on trip having met that standard.

"I can't imagine someone saying that's a marked departure from what a reasonable person would have done," Lerner said.

But it could be altogether different if that person had never entered into the water before, or was terrified of the water, and that was why they failed their swim test — and they were then allowed on a canoe in the middle of a lake, Lerner said.

"So there are all these different factors that are in play," he said. "And that's what's going to have to be dealt with in a court, obviously."

Jeremiah's death also led the province to review the outdoor education policies for every school board in the province.

'High threshold'

Weinstein said it's a "high threshold" to prove criminal negligence. For example, if a motorist is speeding or speeding with alcohol in their system and causes an accident, that often isn't enough to be charged with criminal negligence.

The fact that Jeremiah drowned on a school trip is not necessarily proof that Mills was criminally negligent, even if he was aware that he couldn't swim, Weinstein said.

"You have to look at what are the circumstances that led the child to swim. Did he fall in the water? Did [he] jump in with the teacher watching? We don't know. That's speculation. So you have to look at all those facts."

Criminal negligence cases are difficult, in part, because, to be considered guilty, "you can't just do a criminal act; you have to have a criminal mind," Lerner said.

"And one of the biggest struggles in criminal law is ... how do you balance the need for having a criminal mind in a negligence case. And that is not an easy answer," Lerner said.

He added: "If you look through the court judgments, the courts have somewhat been all over the place."

Some recent high-profile cases involving criminal negligence: