The sixth form is currently about ~100 or so? Each of my classes tend to be anywhere from 10 to 20 people for the STEM Subjects, but that's slowly rising over the course of the year. There's almost like three factions I'd put people into, a lot of us are UK based who struggled with mainstream education, but it's quite an international school. I know a few people from other parts of the world, such as a friend I got into contact with from Germany, a few from Spain, one from Norway; its really diverse in that regard ^^
There's quite a few athletes as well, from my experience other students have commitments like tennis or motorsports; they're working away in the background, and if I had to guess its because MVA is so flexible with every lesson being recorded and the flipped learning model... Of course, its encouraged that you make it to as many lessons as possible, but if you get in contact with your teacher or the attendance team beforehand, as long as everything is handed in and settled by the end of the two week blocks, it's not usually an issue.
The social aspect is quite a nice warm environment from my experience; I don't know if we still had it but at one point the head of year put out a google spreadsheet where we could all add things like what we're interested in, any email contact information, and it was open for anyone who wanted to network to find people with similar interests - I didn't really expect it to work out for me, but it did and with being in sixth form you're treated a lot more like adults, responsible for getting your work done and in time but also supported? It's a weird dynamic which is quite hard to explain, but on the whole the interactions with staff are friendly and professional, and I've not really ever had a problem with them. Of course, I can't speak for absolutely every teacher in the school, but if I had to summarise? Its really easy to approach staff and most other students which is welcome!!
3 - I've been in quite a few extracurricular things in my first year here, including helping run year 11 open days and getting involved with hosting games for the LGBTQ+ club which tends to be a good time, and I think they're currently trying to get sixth formers involved to help run or start clubs, so if you're looking for that type of thing, then there's quite a few opportunities

*Additional edit here, each student also gets a mentor, which you get a ~30 min slot with once a week to help check up on how you're doing. I was skeptical at first, to be completely honest, but I've found it can be really helpful if you have any concerns; almost like a friend to reach out to let them know if there's anything which they can do, such as let teachers know certain things and answer any questions
(also sorry for the wall of text...!!)
Hope you're doing alright!