Technically, it's probably the government providing too little funding to school boards to cover hiring enough qualified teachers. But you're right; this is clearly a structural problem:

More frustrating, she says, was learning that schools in the anglophone system are still short by 32 teachers — and three districts of the four are relying on 132 people on local permit contracts.

Speaking as a former teacher, this is also the result of the increasing stresses in the profession. The pattern for the last couple decades has been to continually load teachers with more responsibilities while providing less support, and many who are trying to enter the profession are burning out and switching to other jobs. I can't imagine many of those graduates would want to come back without a significant increase in compensation or a change in the culture.