A shortage of supply teachers at the Thames Valley District School Board is at a crisis point, union officials say, with students having to forego learning if their teacher calls in sick.

In fact, the list of available supply teachers is less than half the number the board should have, said Craig Smith, president of the local Elementary Teachers Federation of Ontario, which represents 3,400 teachers.

The school board disputes the union's numbers, saying it has hired 217 new supply teachers since September, with another 100 hired in the last week.

"It really is a crisis. It's been going on for longer than a year but really hit a critical point in September," Smith said.

"When a teacher is away, there's no one to take their place."

The supply list, which teachers use to find replacements if they have to be away, has about 800 names on it, Smith said. It should be closer to 1,700, he said.

The board says the number is closer to 1,400 as of Thursday.

The union filed an official grievance in October with the school board. Getting nowhere, it applied for arbitration on Wednesday so a third-party can look at the issue.

CBC News spoke to a school board spokesperson and a superintendent on Wednesday about the issue. Both denied there was a supply teacher shortage.

On Thursday, the board said it's working aggressively to hire new supply teachers, often conducting interviews on weekends.

'A perfect storm'

The London District Catholic School Board says the lack of supply is such a problem, there's talk of hiring non-teachers to fill in during emergencies when no one else can be found according to superintendent of human resources Jim Vair.

"We've cut back on professional development that takes teachers out of the classroom as one way to deal with the problem. We're definitely experiencing challenges," he said.

"There's a problem with supply and with demand," Vair told CBC News.

At the Catholic board, there are 360 people on the supply list. The maximum possible is 700, he said.

This year, there was a 'perfect storm' that created the massive shortage at the Catholic board:

An increase in retirements or teachers otherwise leaving the system.

A spike in enrolment meant a sudden hiring blitz for teachers and support staff at both boards. Those hires come from the existing supply-teacher list.

After years of graduating too many teachers candidates who couldn't get jobs, Ontario reduced by half the number of graduates two years ago. That's meant fewer teachers graduating and fewer candidates to fill the supply list.

Similar issues are at play at the Thames Valley board, the union says.

'There is a problem'

The elementary teachers' union has been asking the Thames board to fill the vacancies since the beginning of the school year, Smith said. The problem is "chronic, ongoing, and unacceptable," the union said.

"I don't know if savings is a driving force, but that's part of it," said Smith. "There's no shortage of teachers in the province. They haven't disappeared. We have constantly pushed the board to be more aggressive to plan for occasional teachers. They have a plan that's very incremental, but the problem isn't incremental, it's immediate."

Lynne Griffith-Jones, Thames Valley's superintendent of human resources, told CBC News on Thursday that "the safety and supervision of students, and their academic progress, will not be impacted by this issue."

But on Monday and Tuesday of this week, the union office got 200 phone calls and 800 emails about problems with finding supply teachers, Smith said.

There was an enrolment increase at the Thames Valley District School Board this year, and the board said in September it would hire 1,800 teachers and support staff.

"The fundamental impact it has on student learning is the thing that galls all of us because there's a lot of lip service about how much the (school board) cares about kids and yet, on any given issue, they seem to come second, and this is one of them," Smith said.

When a teacher can't find a supply to take his or her place in the classroom, students are split into smaller groups and placed into other classrooms. There, they work independently and not on the program their teacher or a supply teacher would have given them, Smith said.

"It certainly has an impact on student learning," he said.

The union has asked the school board to require less mandatory professional development so teachers aren't taken out of their classrooms — a move the Catholic board has already made — and have advised their members to not participate in voluntary professional development until the issue of unfilled vacancies are resolved.