Another “grade two” here.
But if I did use “second grade” I know I would skip the article because I say “in hospital”.
Depends on the construction of the sentence. I can see myself saying it either way.
My son is in second grade. It means he’s reached a certain grade level.
My son is in the second grade. It means he’s part of the group of kids in that school that makes up a class.
The second grade went on a trip.
The word the makes the second graders an individual group called a class.
Yes, and in Australia for the high-school grades, I say, “She’s in year seven.” (My daughter is currently teaching year twelve drama, and is about to get a year twelve English class, in NSW).
Ditto, from Louisiana. I posit that both uses are A-OK in any and all American English usages.
One of my friend’s mother, from Michigan, says “stand on line”.
I’m sure I have used both, but when I thought about it, it seemed more likely that I would use “the…”
That makes it sound like she’s a passenger on a boat, or something.
She’s in second grade.
I stand in line or stand on line; either one seems fine and I have used both.
I always said “second grade”. Putting “the” in front of it sounds vaguely rural, as in “I had to plow the back forty back when I was in the second grade”.
She’s a freak! 
I have never ever EVER heard anyone around here say “stand ONline”.