It depends on what you mean by essential. The child doesn't DIE or get injured from missing school. Obviously education is important but it's not the only place where one could find it.
I would say that I would be in exactly the same place if teachers weren't considered an essential service. Sure, there might have been some strikes everyone once in a while but I doublt there would be a multi year cancelation of school........
The only reason this is given essential service status is so many people end up in arms because their free baby sitting service is no longer there. As mentioned, the school is not a baby sitting service.
Heck, even during a strike you could have admin teach a half day once a week and elementary schools could rotate through the grades and send out homework.
Obviously that wouldn't be as good a situation as having full time teachers but I think some lessons in independant learning and dealing with adversity aren't the worse thing to happen to a child.
And when they finally get to university, they get fracked over because they don't know the basic stuff and their foundation would be deemed as weak. So basically, your idea is to forget about the future generation doctors, pharmacists, dentists etc. because highschool and elementary school isn't important and can be run by any moron. I'd say, the government would be stupid if they are shortchanging teachers and the whole school system. These services are essential, because education is essential and it is a reason why Canada isn't a 3rd world country.