What was Serena and Massi relation?

They were boyfriend and girlfriend who'd just split up recently.

In the early parts of the film (when Serena's dad drops her off), it looks like her and Massi are still dating. But you find out later during Serena's chapter that they just recently split up. Both of their parents still thought they were dating so the breakup must have happened right before the events in the film that place.

What MisterMovieGuy said. Also, SERENA told MASSI that they could still be friends. The reason why they didn't tell the parents isn't explained.