What grade is Sunset Shimmer in?

In the first “Equestria Girls”, we see three pictures of Sunset Shimmer donning the Fall Formal crown in Celestia’s office, which would obviously mean she’s a senior, yet she still attends Canterlot High when she should have graduated after the third movie (or so). Is Sunset deliberately flunking out, so she can spend more time with the Humane Six and graduate together with them, leading to her inevitable perma-return to Equestria?

I always just assumed she was older than the rest but stays in school anyway cuz its something to do.

@LightningBolt  
I know, but why so often?

@Flaming Frets  
So often what?

@LightningBolt  
You can do that? Just hang around in school just because?

@Background Pony #9AC7  
I mean that’s what I write.
 
It’s unlikely for her to be 17/18.

I assumed that the high school was one that operated in grades 1-12. That was my assumption.

@Zaknel  
Then it’s not really a high school, is it?

They’re all in a fictional grade between junior & senior. Cartoon characters are just lucky/cursed like that.
 
Think how Bart Simpson has been in elementary school forever. If you are a toon you don’t really age unless something big in the story calls for it/ a what if future, etc.

@Flaming Frets  
Sorry. I was busy. What do you mean?

@Zaknel  
I mean, if they have grades 1-12, can’t really call it a high school.

@Flaming Frets  
Well, that could be called a high school. As could it be called an elementary school and a middle school. Maybe all of the adults would consider it a high school given that they last attended it as that.

It might help to assume that CHS is grades 8-12 instead of 9-12.

@Flaming Frets  
The “Spring Fling” is also mentioned - Sunset did something horrible ot Rarity to win it - so the crowns may only reflect a couple of years. Still, that leaves her only two or, by Vivace’s method, three from that point. Pretty much everything up to Movie Magic fits in one, but the shorts have played havoc with a sensible timeline.