Maybe in the messed up States...not common at all all around the rest of the world. Never heard of it, nor ever seen it in the UK (or in Europe where I've spent a lot of time) and likely would imagine it's illegal here too. You cannot keep someone in school past the age of 16. It's a simple as that. So no, it's far from common.
It has to be 2 years, otherwise the first scene with Iris is still problematic. Which creates other isues...as he dated her for 2 years...now being changed to 18 months (pointless change imo but whatever)...the fact of the matter is... he was supposed to be 5 years younger than Erynn/Alexis (part of the reason she didn't take him seriously and was surprised, he was a fucking kid...
This is not counting the the fact that everyone in school is 18, which is utter bollox. They are not going on spring break, they are a month or two max from leaving high school...you know, the actual legal age you leave school, Nyx seems to have completely forgotten that that what 'year' you are in is directly affected by your birthday. So if everyone is 18, they are due to leave school imminently...they would not be 18 in the Jan/feb of that year because if they were, they would be in the year prior.
So again, 18 when met Iris, dated for 18 months....3 months(?) at Cara/Nikki's.....the MC is almost 20 years old when he returns. Changing the MC's age requires a complete script rewrite of many, many things, again, all because Nyx thinks she is far more intelligent and smarter than she really is, and wants to continually walk along the white line of acceptability.
Bro stop huffing paint... I'll just echo what
HiP1 said about the french school system, because I went to a french school and I know for a fact that students with low grades are (or at least were) advised/encouraged to repeat the penultimate highschool year if they had low grades, I was advised to as well but I told them to eat shit since I already had to repeat one year because of having to switch school systems, but there were many students that did... If I had chosen to repeat that year, I would've been 20 right after finishing highschool... And even with that, when I got into college right after, at 19, I was the youngest dude in my class so make of that what you will.
So I guess your "worldly knowledge" is shit.
Secondly, he didn't stay at Niki & Cara's for 3 months, it was less than a month and a half. The first flashback when Andres tells MC about getting married is clearly marked as "2 months ago", then "2 weeks later" he gets his last tattoo from Lakeyn and the next day they leave for Riverside... that leaves around a month and a half before the game starts.
By the way, the most unbelievable thing about the game is how fast he gets his tattoos... he gets that huge side tattoo in one session, and he even fucks Lakeyn after, which would be probably impossible due to the pain and the risk of infection... speaking of shit that doesn't make sense.
That said, yes, she's gonna need to revise a few things with Iris, probably, a couple of dialogue changes so it makes more sense, but they could've dated and then fucked later on when he's 18, point is: just needs a few tweaks, not a full rewrite... just changing the relation with Iris from 18 months to 8 months would be more than enough, and honestly, a minor change.... 8 months is still a shit load for a relationship at that age.