We had Grade 13 (then called OAC, which stood for Ontario Academic Credit) when I was in high school, and what I found useful about it is it gave us an opportunity for greater independence within the high-school context.
About three quarters of Grade 13 students were 18 years old when the school year started in September, and everyone was 18 by the end of December. This is relevant because 18 is the age of majority, students over the age of 18 weren't subject to the same rules about care and custody (for lack of a better word - I think there's a specific term for this but it escapes me).
In practical terms, this meant that we could sign ourselves out of school rather than needing a note from our parents and we didn't need our parents to sign report cards or permission slips. In fact, our teachers were not legally allowed to meet with our parents without our permission!
OAC classes operated on the assumption that all their students were over 18. In practical terms, this meant that if the teacher was absent, class was cancelled rather than having a substitute teacher. There weren't any parent-teacher interviews. We were expected to manage our own education and our own time like adults.
At the same time, the societal expectation was that we were still high school students and our parents were still expected to care for and support us as such.
If a parent had kicked an 18-year-old high school student out of their home, other parents - even those who would have responded positively to kicking out an 18-year-old high-school graduate - would be just as appalled as if they had kicked out a 17-year-old
Young people who would have responded to a peer saying "I'm an 18-year-old first-year university student and I moved out of my parents' house!" with "Cool!" would have responded to "I'm an 18-year-old high-school student and I moved out of my parents' house!" with "Is everything okay?"
This meant that we were empowered like adults to manage our education and our time, without being expected to take on the full suite of adult responsibilities like paying bills and buying groceries and managing a household. It was a sort of training wheels for adulthood.
It also helped train our parents for our adulthood. Our educational structure moved away from parental permission or parental involvement even while we were living in our parents' homes, which prepared our parents for not having direct involvement in our post-secondary education - something that's even more important today when it's even more financially difficult for students and young adults to live independently from their parents!
I had an on-campus job in university when Ontario eliminated Grade 13, and I noticed an immediate difference in parental involvement when the first Grade 12 cohort arrived. Parents were contacting us directly or accompanying their kids in person even for mundane things like asking how to configure an email account, seemingly without any attempt by the student to do it independently. I was only a few years older, but that simply wasn't done in my cohort!
So if they do end up re-introducing Grade 13, I hope they take into consideration that Grade 13 students are going to be legal adults, and create a system and structure that reflects that, rather than a system and structure that has young adults spending their first year as a legal adult being treated like a child.
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Even though my own actual firsthand experience with Grade 13 is that it was positive and empowering (and even though my own actual firsthand experience is that I felt too young for university in my first year, even though I could handle it perfectly well), I think if I were a student who expected my high school only to go as far as Grade 12, I'd find it insulting that they want to keep me in high school and living with my parents for another year.
Similarly, if I were an adult who had graduated high school after Grade 12, I'd feel insulted on behalf of the youth of today and tomorrow that they'd have their launch delayed another year.
This is why it kind of surprises me that they'd put this in a platform with the presumed goal of winning votes for an election. I'd imagine there's a significant segment of the population who would see it as completely unnecessary and perhaps even verging on punitive - especially since it has always been possible for students to keep attending high school if they aren't able to graduate or get the courses they need in the allotted number of years (historically this has been called a "victory lap".)
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A caveat: I've noticed in recent years that teens and young adults (or, at least, a big enough proportion of the teen and young adult voices that reach me for me to notice) seem to perceive being considered/thought of/treated like a child (as opposed to an adult) as more positive than I do.
They seem to feel that if you're treated like a child, you're being protected and cared for. Meanwhile, my experience - even in retrospect - was that being treated like a child meant my agency being disregarded, with no increase in care or protection. (And often, in my experience, "care and protection" was the label given to disregarding my agency.)
So, because of this, it's possible that today's young people might not feel liberated by being treated like an adult as opposed to like a child.
However, I am also aware that adults all too often will read or hear something about Young People Today and use that to treat young people with less agency than they should. I can't tell whether I myself am falling into that trap.
So, as with all aspects of life, the important thing is to listen to the people actually involved - today's high school students and recent high school graduates.