To Heart 2 is the future of Battlestar Galactica
Knowing that BSG takes place 150,000 years before our time, the HMX series are the new cycle by being to modern humanity what the Cylons were to the past humans, hence why they are Ridiculously Human Robots. Or perhaps they might even be new humanoid Cylon models...Marianne knows Aoi Matsubara and Kotone Himekawa from the original To Heart
According to the Visual Novel, Marianne was held back by one year because she wanted to stay with Sasara in school. It was already stated by Word of God that To Heart 2 takes place three years after the original series. By doing the math, with Marianne having spent four years in the school, that means that she was a first year student when Hiroyuki and Akari met both Aoi and Kotone, who were also first year students.