I’m in my late 40’s. I can’t even tell you what my low grades in high school were. I was first gen female. My parents were happy with my B+/A- average. I took the ACT once, zero prep. Did ok. Sure I could have done considerably better with a little more exposure. Did much better for grad school testing.
1 - your English teacher is right. No one cares.
2 - I’m not sure my in-laws know what degree I have from college let alone anything from high school. LOL.
I think the beautiful thing about our society is there are many chances to redeem and reinvent yourself. There are higher ed options or those who completely bombed high school. I have a friend who was a college failure and drop out who became a straight A student in her early 30’s.
I wouldn’t worry about an employer asking about your SAT scores/high school grades over the long term. It says right in that linked article it’s not a common practice.