A common misunderstanding of the first movie is thinking that the "3 previous coronations" of Sunset Shimmer in C.H.S. correspond to the same event ( Fall Formal ); while it is clear in the subtext that the "Spring Fling" is the "Last Spring Debacle" mentioned by Rarity near the end (also foreshadowed by Applejack using baloons earlier).
The "Mirror" opens once every 30 moons (so once every 2 and a half years). The "humane" mane five were happy as freshman students. Sunset Shimmer was also likely a "nice & proper" student on her first year [she did not participated (or did not won) on the first Fall Formal]; only becoming "an evil Alpha Bitch" after two consecutive victories [ one Spring Fling & one Fall Formal ] when she had to sabotage Rarity´s circle of friends in order to prevent them from convincing most of the students to vote against her in her second "Spring Fling".
There are no reasons to believe that "Pedestria" (the world of C.H.S.) & "Equestria" run at different timescales.
Between the first EQG movie ( "Through The Mirror" ) & the second ( "Rainbow Rocks" ) only a few months had passed [this is also the entire duration of the "unfolding events" proper for Season 4 of FiM, since many of its filler episodes were "recycled" from scripts originally made for the batch going from S3E14 to S3E24].
A similar "timeskip" goes between EQG 2 & EQG3 (Friendship Games), as the time it took for the Sirens to "track down" the source of the Rainbow Blast of the first movie [using their "magic sense" (initially) and their hypnosis on random people (later) to get its location], is what Sci-Twi needed to pin down the stronger "magical signals" from the Dazzlings/Rainbooms battle with her equipment.
The EQG4 movie happened AT LEAST an entire month after the Friendship Games (given what Applejack said about being "waiting" for a non-magically compromised fun activity with her friends). Since Camp Everfree is open only in Summer, it means a bit less than a year had passed since EQG1, that was precisely at the begining of Autum ("Fall" Formal).
The "Mirror Magic" EQG special "syncs" with FiM by having Starlight Glimmer at Twilight´s Castle in Equestria bored (while the Mane Six go "on a cutiemap mission" without her); meaning it happened AFTER FiM_S6E12 "Spice Up Your Life" [when Princess Twilight & Starlight fixed the cutiemap]; but before she rescued them from Chrysalis in the Season 6 finale (given the "lessons" she learned in that special).
The "beach shorts" happened on that same late summer; including the "Forgotten Friendship" EQG Special. There is a posible connection between Celestia "unlocking" the Restricted Section of the Canterlot Library for Sunset & Princess Twilight; and the latter´s interest on reading obscure 1000+ year old texts in Old Ponish (that served for the plot of the Season 7 finale), likely around the end of Fall (on the side of "Pedestria").
The last known "proper" crossover is on the EQG Special "Spring Breakdown" (happening obviously in Spring), after a very long timeskip of many months [there was an Equestria Girls short with humane!Rarity having to deal with cold & snow]. Spring Breakdown is canonically after the MLP MOVIE 2017 (somewhere in between the "coldstart" of S8E01 and the rest of that episode), given that the School of Friendship was not built yet.
The FINAL Equestria Girls connection (debatable) was on the "Sunset´s Backstage" EQG special; when Princess Twilight mentioned that "recently" the magical device that Sunset was looking for (timeloop) had dissapeared from its last known location in Equestria. The only logical reason for Twilight to get this information would be if she was checking out for OTHER artifacts stored in the same place; something she did just before S8E14 ("A Matter of Principals") as preparation for her "magic hunt" event [that was followed by a "winter" episode].
The S8 finale happened BEFORE the end of the (second?) "semester" of the School of Friendship. The S9 premiere was concurrent with the "summer recess" of the School of Friendship & in S9E06 it was shown that Fluttershy was the "Best Teacher" for more than a year. Starlight & Trixie´s special "Equinox Cake Party" [S9E11] was likely for the event marking the begining of Autumn (equivalent to the "Fall Formal" in Pedestria). The FiM Special "Best Gift Ever" is likely placed in the next winter, followed by S9E17 ("The Summer Sun Setback") at the end of the next Spring...
-> Princess Twilight´s actual coronation (sometime after the S9 "finale") may had happened at the same time than another "Fall Formal" in Pedestria.