Some of the characters shouldn’t have been in highschool at all.

To those questioning the other ponies being at the mirror right as Twilight and Spike came back remember that this was moments before the portal was supposed to close so it’s possible that they had left and came back since they were already told how long the portal would stay open.

That, or ‘30 moons’ means something totally different in a world where the moon has to be moved manually.
 
Like I said back when everyone was trying to figure out the ‘100 moons’ thing from Apple Family Reunion and claiming that it meant the first half of season 1 took place over the course 8 years.

@ClownDicks  
Well, they cared about the characters, the sound and animation, and they threw in a few jokes to the older fans (Scoot does the chicken). It’s this timey-whimey stuff they didn’t care about, since it really wouldn’t affect the plot.

I still don’t get what OP was implying. Assuming that the EqG counterparts are all 17/18 years old, the portal has been open:
 
Senior year of high school  
Sophomore year of high school (possibly when Sunset Shimmer first came)  
8th grade (another possibility, though Shimmer would have at least waited until freshman year started)  
6th grade  
4th grade  
2nd grade  
Kindergarten
 
It doesn’t open very much. I almost think that she got into the portal when the counterparts were in 8th grade and jumped back whenever she could to do God knows what before jumping back again. But of course they’re going to be out of school.

All of this can be easily explained: absolutely zero thought was put into Equestria Girls because the writers didn’t want to do it and halfassed it as much as they could without getting fired or pressured into quitting.

@Mayojar77  
A few days open then 30 moons before it opens again. It’s never said how long that is in days or what it would be (if different) in human time.
 
Celestia also implied that she’d been watching the mirror for Sunset’s return and this was apparently the first time she’d been back.
 
Gah, I’d like to debate how time works (or could work), but I doubt the writers really cared enough about that and just gloss over it with vague terms. They know the target audience won’t care beyond “short time limit” and “long wait penalty.”
 
@Mayojar77  
Hmm, maybe there’s an adult Sunset wandering around somewhere. That would conveniently remove her from the high school setting.

She won’t be going back, the human world has their own Twilight.

@Background Pony  
I think so. Equestria Girls: COLLEGE EDITION
 
okay, I need to rephrase this title.

Might the 2 1/2 years be referring to how long till the gate opens again?

@Mayojar77  
The ages in the movie are messed up anyway, no way are the CMCs only three years away in terms of age from the Mane Six.

@Itsthinking  
It essentially means that Sunset couldn’t possibly be the same age as Sunset unless it wasn’t the real Sunset.

@DoomPony  
They really haven’t shown or explained how their perception of time works in Equestria. I just now realized that. holy crap.

@DoomPony  
What’s stopping them from having left at different times to go do things while Twilight was in there? Ponies aren’t static, and Celestia apparently says there’s a time limit.

@Mayojar77  
I don’t doubt that she is.
 
@DoomPony  
She was there a little over two days. Got there on Wednesday afternoon, left Friday night.

Wow, I type slow.

This assumes that time functions identically for Equestria and Humanland. Come to think of it, we have no concrete information on how time works in Equestria.  
How many days is a pony year? Granny Smith is potentially 300 pony years old.  
Twilight only spent about a day and a half in Humanland, yet the mane six five and Celestia were still just standing there when she came back. How long was she gone from their view?

@Itsthinking  
Or, they could have left and come to check up on her every now and again. They do have lives.  
@Itsthinking  
That still leaves Twilight much younger than Sunset.

@Mayojar77  
They stood by that portal for three days?

@Mayojar77  
yeah, it won’t make sense. Twilight would’ve come back years after she left for what seemed like a day.

@Mayojar77  
Yes, and she quit. Shimmer wasn’t a brand new student like Twilight was. The EqG counterparts had known her for at least four years by that point. She’s gone through the portal several times. One would assume that she was under Celestia’s care for a short while, gave up, and started jumping dimensions to plot her revenge.
 
She really didn’t do any magic until she got the Element of Magic. It wasn’t magic she was good at, it was social manipulation that caused her to control the school. When she finally put the Element on, it physically hurt her. She was crying from pain.
 
My headcanon is that this shows that it’s more powerful than things like say the Alicorn Amulet. Anyone can wear that. It leads to slow corruption, but anyone can wear it. The Element of Magic/elements in general must react violently to not being worn by their intended bearer, or someone with an impure heart.

@Background Pony  
From what I heard, it’s all in real time.

@Background Pony  
I doubt it.

@Mayojar77  
Possible time difference in alternate worlds?
 
like time goes slower in the other world or something.

@Sephiru  
I’m guessing from context that there’s a passage-of-time disparity between Horse World and Planet Of The Apes that gets explained in the movie, possibly because she spends several days in Monkeyland but everypony acts like she just left seconds earlier once she’s back in goddamn Friendship Is Magicuestria where she belongs.