I work at an independent school. I also send my older 3 to independent primary schools. I attended state school and have taught at state schools too. DH has also taught at both independent and state (all just to say I know th3 system!).
You have concerns. So raise them with the school. There are staff willing and available to talk to you.
WhatsApp group: get him off it!!! Schools hate it when these get set up. But its difficult to prevent outside of school hours. They are a pastoral nightmare. Get him off and please alert the head of year/section and they will almost certainly put out the word to the kids/other parents that it shouldn't exist. They won't be able to totally stop it totally though. These things are particularly in issue on school coaches and similar where children are less supervised than either at school or at home.
Issues getting on the coach: ask about which staff are responsible. There'll be someone supervising (may not be the same person every day though). If a lot of kids get on the coach it can be a bit of a scrum! Look up the photos of who is responsible each day and make sure you know DS and ask form tutor to share a photo of your DS with them and ask for particular help to be given for the first few weeks. We get lists of who needs a bit of extra monitoring.
Clubs: have they all started? At ours sports starts straight away, then music is added (which we've got to now), most other non-academic clubs start next week. Academic ones even later for the younger ones, usually around half term - they need time to settle into their lessons and are prone to over-committing themselves. We also need to get a better idea of their strengths and weaknesses and ability to organise themselves. But lists of what kids/parents have signed up for went out to form tutors yesterday so we could start having conversations with kids if there seems to be too much/too little etc. If there is something missing from the offering you expected RAISE it. Perhaps the staff member who usually offers it has left, or perhaps there hasn't been enough demand for it this year. But you wont know unless you ask. Email form tutor and they'll pass it on to the CoCo lead if necessary. Equally, numbers attending clubs may pick up as people are directed towards them. They may not - we have dome that are very small but they become a pretty tight knit community (our senior level board games club is a safe haven for several pupils thay struggle socially. Some have been attending for 5+ years and they are all incredibly supportive to each other)
The school day IS longer. Especially if you take the coach. What would transport be like if he went to state? Or could you sometimes pick him up to give him a shorter day and you more time? You will have longer holidays of course - make the most of that time together if you can (which probably depends on your working situation).
For those saying a smart child will do well at comp or private... well, that's true for VERY bright children. Statistically it isnt true for the average child. That's not meaning any insult to either the child (I was one, state throughout) or the teachers at state schools (I've been one, my DH is one), but the resources at an independent school are much better because there is more funding. We have the resources to target pupils who might just miss an 8 or a 9. There is less classroom disruption, partly due to selection, partly due to the ability to remove disruptive children, partly just that we can have much smaller classes when needed - the top GCSE sets are about 24 kids, but lower sets (where there is more disruption) may only be 9 pupils. This means both that those pupils just arent affecting many others or taking up an unreasonable proportion of my time, but I can also offer them attention and support, which they need.
Independent education is VERY expensive. There are many other things you could spend your money on (tutors, music tuition, wonderful educational holidays, deposit for first house....). Only you can decide what is right for your family. But if you are going to spend the money TALK to the school and get what you are paying for. Mumsnet cannot fix any of these issues, but the head of year might well be able to. They are there to support you and your child but arent psychic.