Yeah, I've suspected for awhile that the timeline for all these relationships is jumbled up. I think all the holiday events are taking place concurrently, we just see them way out of order.
That would make a lot of sense. It's pretty much the only logical way of doing a very long running manga in a school setting, unless you go the comedy route and none of it really matters a la yuru yuri.
Mind you, the timelines still doesn't make sense. For instance, Honoka first meets Ed and Ian when Eli invites her to their home for Christmas. The next time they meet is during the summer festival. As I said above, the school year ends in March. The previous chapter confirms Eli won't stay in Japan long after the school year ends, since it's set a month before her leaving and Kaoru still hasn't graduated.
As far as I can tell, the time rules of this manga are something like this:
1. Chapters can take place whenever in the school year. Despite this, each storyline advances linearly. For instance, Chie and Ai are not dating in any chapter that appeared before ch. 105 and are dating in all chapters that appear afterwards, unless the chapter is clearly marked as a flashback.
2. To avoid really blatant paradoxes, no character is allowed to celebrate the same holiday or other event twice.
3. Entrance exams and running out of time with their romances because of graduation are not usually a concern to any of the third years. EliHono is the only couple on a deadline, at least for now.