EQG Ages

Theres a topic I've seen a lot of analysts and youtubers mention when they discuss Equestria Girls, and that is the ages of our mane cast. I'll admit, it is odd for characters who live alone and have jobs and are considered adults in the show to be in highschool in the movies. I have a theory though. I think that it has to do with the different cultures.
In Equestria the schooling is clearly different. It doesn't focus on high level math or anything, but rather on history and finding your cutie mark (from what I've seen). From the looks of it, after you get your cutie mark you then either go into a higher level schooling pertaining to it, or you finish the basic schooling.
Let's assume that since MLP is made in america that the school system and culture are for the most part parallel to the American one. Children have many more years of schooling to go through before joining the working class, not to mention college.
Therefore I think its safe to assume that the reason the main cast are in high school in the Equestria Girls films is not from an age change, but rather a culture change. Ponies are considered adults at an earlier age than their human counterparts. Its not too far a stretch, after all Rainbow Dash did win the YOUNG fliers competition.