I would tell you to be prepared to meet a crowd more international than any you've likely encountered before. Speaking as an American, I had never truly interacted with people my own age from other countries. By the time I left Reach, I had friends in San Francisco, Belgium, London, Iran, Austria, and Greece, many of whom I am still in contact with. This allowed many fascinating discussions where we compared our cultures as well as our school systems.
Breakfast starts between 7:45 and 8:00, and classes start at nine. It can take between zero and twenty minutes to walk to your class, depending on which college it is in. The classes then run until one, with a few fifteen- or twenty-minute breaks. Most afternoons, you are then completely free until dinner starts at 5:30. There are afternoon activities, such as sports, but most weren't mandatory.
Many days, though not all, there was a lecture in the evenings, which lasted from 6:30 to roughly 7:30. There were then evening activities, such as the talent show or the charity auction. On nights when there were no compulsory evening activities, Reach was usually still running something like Wii sports or a sing-along competition, to name a few. Curfew is at 10:30, but you're supposed to be back in your college at 10. It is then entirely up to you when you go to sleep.
I suppose my biggest fear was that I wouldn't make any friends and I would have to spend the program alone. It was sort of a nagging fear in the back of my mind up until the first day. As it happened, I did meet three people on the first day with whom I immediately connected, and over the next few days, three more joined our little group. However, even if I hadn't met those, Reach was a uniquely welcoming environment.
Breakfast and dinner are both eaten in a dining hall, where everyone sits at one or two long tables. This means that you couldn't sit alone even if you wanted to. Because everyone is let out from classes at the same time, there is a good chance someone would invite you out to lunch with them. I found the kids at Reach to be extremely socially proactive and inviting. I can only assume that this will also be true of future programs.
Reach students will stay at one of two or three different colleges. Both colleges and rooms are decided based primarily on age group. I stayed at Downing, where the younger students were. There were about forty five kids between 14 and 17. The older students, the 17- and 18-year olds, stayed at another college.
Neither your course nor your nationality has any effect on where you stay. My roommate happened to be taking the same class as me, but that seemed to be unique. Corridors are single-sex, but the staircase as a whole is co-ed. My staircase had boys on the first two floors and girls on the third. You can also put in a request for your roommate if you know someone else who is going, or you can request a single room.