@Whatwouldscullydo honestly? I think there are too many strands to unpick:
Social services need more funding - they are massively understaffed (our local council only has 2 safeguarding SS employed permanently and they've been advertising the jobs for about 5 years, no one wants to do the job because the pay is peanuts and there's very little value added). Because SS are so stretched, schools (and the police tbf) are having to pick up the pieces for free. More time spent dealing with stuff we are ultimately not qualified to do means less time to do the stuff we should be actually doing e.g. in my RQT year I was sent to do a home visit on a neglected child because SS couldn't get there for a while as everyone was working on a full caseload. My classes for that afternoon had to have a non-specialist babysit them whilst they did cover work.
Outside agencies e.g. CAMHS need more funding and we need to be in a position where we can give students outside support. The number of kids with SEN is going up and up but funding is going down, so they are not getting what will help them thrive. Again, this is coming down to the classroom teacher who then has, as you say, many conflicting needs in one classroom (I regularly teach classes with student with ADHD whose parents claim I need to stimulate their child more to "control" them, alongside a student on the autism spectrum, for whom too much stimulation makes them have a meltdown.) The level of differentiation I have to do to help my students achieve, which I want them to do, is like a full-time job now on its own.
For the reasons above, and as per PP, we need to be paying TAs more to make them even apply for the job in the first place, before we even think of retention, because again, you can't give a student 1-on-1 support that their plan asks for, if no one is applying for the job.
Get rid of the uniforms and have a blanket dress code like most 6th forms and European schools do e.g. no crop tops, no logos, no references to guns etc. It does not avoid bullying because they still know who the poor kids are and find something to bully them about e.g. shoes, bag, the faded school jumper because it's 2nd hand, and I've dealt with several cases of bullying because they have a stitched on logo on their blazer rather than the usual school one. This will help parents make school clothes more affordable which means they will have more money to put into affording living cost price hikes, school equipment and revision materials.
Sort out the way staff are performance managed and what kids are being examined on, so that teachers don't carry the whole blame for students failing. If a student with poor attendance, lots of SEMH issues etc. is failing, it's probably not for lack of the teacher trying. If teachers know their ability to pay the mortgage next year is based on needing 80% of their GCSE class to get their predicted grade, based on exams they took under very different circumstances when they were 10 years old, then the only thing that will matter will be dragging that grade out of that kid, which is when you get parents complaining about lack of focus on mental well-being. You also have the dreaded parent saying "I'm making my child concentrate on the subjects they need to get onto their course at college" - which is reasonable, except that will impact whether I get a pay rise next year and whether I can afford to eat 3 meals a day. Equally, the exams are very skewed to certain demographics. One paper talked about a winter sports holiday, in the school I was in at the time, the kids' version of a holiday was if they got to go to a caravan over the bank holiday weekend, they couldn't ask questions to the ski instructor because they had no idea what that would look like. So they weren't being examined on the subject being taught, they had the added disadvantage of being too poor to understand the context to draw experience.
And this is just the tip of the ice-berg. We haven't even got to schools funding meaning staff are being let go, classes made bigger and timetables stretched with more cover. The amount of admin needed to "prove" what we are doing just in case Ofsted come, which has no benefit to the students. And I'm sure there are also many other layers that I can't think of just yet.