In HS, I think it mostly assumed that you take your classes and pass them, and move onto the next year. I know I never worried about how many credits I had in HS. That was the major difference between HS and college for me. Students tend to rely on advisors to help fill out their schedule and what classes to take in college, HS is much more scripted as far as what is to be taken each year. I agree it is a weird story, but why didn't someone help the kid get all the credits he needed?
The weird part is that it's Tabor. Elite Prep Schools drag kids, kicking and screaming, to academic performance. For 40 - 50K a year, that's what parents expect. A friend who went to Choate once told me that it was almost impossible to flunk-out. He did, however, manage to get kicked out.