I have no problem with the age, as I'm aware that in the US, high school student seniors turn 18 or 19 in senior year? I think its 18, so D spending two weeks with F could be during one of the holidays between semesters, if that was the locale.
On the other hand, here in the UK (And dont think I didn't notice london underground signage at one point!) high school runs the five year groups of 11-12, 12-13, 13-14, 14-15, 15-16, with "Sixth Form" basically high-school operated college running the two year groups 16-17 and 17-18, though if you're not doing basic, english math IT or whatever for A-Levels, but specific courses, that's when you go to a proper college instead. Here, University is separate from college, where-as in the US I believe they're both the same, sometimes called both? aka UCLA, "University College of Los Angeles", etc.
So I can see where others are coming from.