Rean isn't as caught up as everyone else because of the time he was in Crossbell, so he's staying for the full 2 years (although punctuated by special missions for the government, I guess), but they never properly explained exactly why the rest of Class VII couldn't wait and graduated a year early.

That was what I got out of it too... the explanations weren't thorough, but you get enough to have a sense for why most of them decided to leave a year early.

• Emma was really just there on Hexen witch business. Once the Awakener business was finished with, she didn't really need to be graduate of an Erobonian military school.

• Jusis needed to take over managing Albarea stuff. Probably not easy to manage an entire province while also being in school fulltime.

• Elliot decided to go to music school full-time. This was a bit of a surprise given that he seemed pretty happy at Thors, but his happiness is only contingent on the entire class being there. Once the class split, his decision to leave makes sense.

• Machias didn't have a real reason to leave iirc. He probably would've benefited the most from actually finishing school before returning to Heimdallr.

• Gaius could've stayed.

• Laura could've stayed.

• Fie was off searching for other Zepyhr members. As a Jaegr-turned-student who was recruited by Sara, a formal education probably didn't hold much more appeal for her and it seemed like she went the bracer route anyway. Plus there was some urgency with the Zephyr boss.

• Millium could've gone either way, but since she only went there because the Ironbloods were investigating C and with Osbourne returning, she probably was busy with new Ironblood stuff.