I agree with lots and hope to get back with specifics later.
I know that HADD (an ADHD support group in Ireland) have great tips on their website and a booklet for teachers (separate ones for parents).
www.hadd.ie/education/teacher-tips
DD has problems with loud noises and very unstructured things, so yard/pitch, drama and group work can be difficult for her. Some teachers have been better than others - letting her have a few minutes away from the bustle, or sending her to secretary or principle's offices with a message etc. She often has a seat in the table nearest the door on purpose, and is allowed to be at the back of the line (although she always chases to be at the front). When she had a teacher not so in tune, we were hearing that she was spending periods under the table and locked in the bathroom - at least she was pulling herself away rather than having a full on meltdown. So letting them have a space if there is a need to, helps.
Agree about specific instructions. And the potential for "just a minute" comments to be timed!!
If you can, get to meet the child (and parents if poss) either before school starts and talk to them about what helps them, or get a few minutes in the first day or so to touch base with them specifically. Let them know that if there is a problem like XXX or X, to talk to you about it. You cannot always fix it, but you want to help everyone learn and enjoy school. (Or something along those lines that fits).
Warnings slightly before activities change that you are going to finish up this and move on to that are great. If possible, and I know it's not always possible, give a warning that a planned activity or lesson will not happen or times for it have changed, before that specific change comes.
Watch the child, when you can, for the first little bit and get an idea of them relaxed and happy, and how they are when more anxious - to get an idea of the things that make them happier, things that stress to prepare more for, and also the behaviours that will give you a short warning that you need to defuse a situation (give them an "out" before meltdown hits). That may be finishing up an activity, giving a slightly longer time to finish it, splitting up 2 or more people, giving the ASD child an alternate task or sending them out of the classroom on a job to give them a breathing space...etc.
And at the same time, acknowledge that the whole room cannot be held to account by 1 child. So let the ASD child know early on as well that you will give them as much help and attention as you can but you need to do the same for each of the children in the class, so there will be times they need to wait etc.
In DD's class, there are jobs that are rotated every week amongst all the children. One teacher occasionally let a non-rota child do some jobs for an easy life if they asked (a kind of "I want to do X", "oh, alright, whatever" situation) - which did lead to meltdowns at home on more than 1 occasion as DD got very upset someone else was doing her job. So if you do set jobs for people, try to let those people do them unless they are absent.
If there are big things happening out of the ordinary, try to give a heads-up on that in advance if you can. Either letting the class know as a whole, or even just a note home to parents to let them know to prep the DC for the following day if you don't want to let everyone know - external visitors, unexpected outings, nurses for jabs, changing of tables groups, absence of normal teacher (and whether that means all splitting between the other classes or a sub coming in), big surprise visitor to assembly that would be hugely exciting but noisy and chaotic, etc. It doesn't always mean explaining in detail, or letting the cat out of the bag about surprises - and some things can't be known in advance anyway - but a chance for preparing the child for something different is always useful. And the parents can often tell if something needs to be said or not - sometimes it doesn't matter but if there are other stresses happening (at home or school), then preparation may be vital.
Also, if you can, have a line of communication with the parents. We hear about some of the achievements, so we can praise them when they happen. We hear a selection of the problems, so that we can work on specifics to solve them (we never talk to DD about things that have happened in school unless we are asked to - but we might work a bit more on a specific thing on "how to share nicely" or "how to tell someone that you don't want to change the rules in the game today but can change them tomorrow" if we hear that there are problems in that area). It ALSO means that we can let the teacher know if there are problems at home - one of us is travelling a lot so DD is missing us, a big row with friend on the street in a different school, upcoming BIG events that DD is getting anxious or upset about (family weddings, ill relatives or bereavements etc - luckily no one directly involved yet, but DD has had 2 GGPs (my GPs) die in the past 5 years so knew them all). So the teacher then knows that there is less tolerance than normal for the everyday issues and also can have a chance to have a few extra things to help out up her sleeve (make sure resource time happens, keeps some jobs/notes to give non-classroom time, keeps a special watch during group work or sets a slightly different task that may be less challenging but still achieves the learning objective, or maybe puts off 1 task until another day).
OK, I think I found time for a few specifics!
We have certainly found that DD is challenging - but school run a "lego club" for half of big break 3 days a week for a good few with problems in yard. The secretary and principle have lots of notes coming from most classes - some of which are just "X needs a walk". And they break up the groups too, so there are DCs wandering all day for different reasons (going to learning support, smaller groups for maths and reading split across the 2 classes per year (so mixing both classes), doing lots of activities in the hall/yard/garden or even wider campus, and bringing in outside opportunities like animators visiting the 3rd level College they share the campus with or visiting scientists or sportspeople). They are mainstream but have a number of ASD and other SN DCs, and plenty of other, non-DX, challenging DCs too. But they try their best to support them all and live up to their ethos of inclusivity and child-centred learning and development.
Its really great when the teachers do want to engage though, and luisten to the parents as to how their DC "ticks", their foibles, their stressors, and how to get the best out of them.