Hi, I'm so sorry it has been so long - I've been super busy with A levels, I don't know if you'll even see this now! I thought I'd reply just in case and hopefully it will be useful to someone!
We all stayed in Van Mildert college, you may be in a different college this year but they tend to go for the bigger ones because yes, everyone is in there together. Accommodation tends to be a couple of same gender blocks and then a mixed one too so people on my corridor were from all sorts of subjects but the people from my subject were always nearby. You tend to sit and do activities with your subject group (there is usually 8-15 of you, depending on the subject, I think there were about ten of us in my subject). You might have heard that van mildert is one of the worst colleges but it was certainly lovely when we were there, the accommodation is relatively modern I suppose, think university standard rooms, but mine had a gorgeous view of the lake and the ducks were adorable!!! In terms of seeing other colleges, You'll definitely have the chance to see the outside of all of them at least once, and we had a dinner one night at Hatfeild so you're likely to see inside a couple, but most are closed up for the summer.
In terms of evening activities, they were all very fun, lots of teambuilding games to get to know the people on your course, we had a silent disco one night, Karaoke, a formal (dress up nice and have nice dinner), a picnic night. Loads of fun stuff and all of the groups mix at night so although you'll mostly hang out with your group you di get a change to talk to everyone. You'll also be meeting others when you do the sports and societies, if that happens this year.
In terms of making friends, it might seem daunting but, literally, everyone there wants to make friends! I made some super close friends over the week who I message daily, and all of us from my subject are in a group chat, most of us heading to Durham in September (results dependent!) so looking forward to meeting up again! Its made a lot easier by having a subject in common you can discuss. Also the time you arrive doesn't affect how well you fit in at all, everyone is super welcoming!! I'm hoping first week of uni will be as easy to make friends! The other thing is the Durham student reps - they are super amazing, you'll have one with your group and plenty of others helping organize it and they all do their best to ensure you have the best time possible.
I'd say about 200-300 in total so the summer school was very busy. The uni was of course quite quiet because most students had gone home so it wasn't overwhelming, but there were plenty of people in the town itself and because Durham is a city uni rather than a campus uni it kind of feels the same.
I hope that covers everything, but if there are any more questions I'd be more than happy to reply, and hopefully with better timing now that I'm finished. I hope you both enjoy the summer school and good luck with your a levels! Maybe i'll see you around Durham in a couple of years!