Hi Kate, how nice of you to come here and ask. I wish my dd's teachers were as interested!
IME there's often a lot of struggle around transitions - starting the school year, starting the school day. I think it's excellent advice to meet the child (and parents) in advance. Is the child at the school already? If so, useful for them to regularly visit your classroom, maybe take some photos of them with you for them to take home and look at over the holidays.
Getting into school can be difficult. My dd copes best if she is given an immediate job that brings her into contact with the teacher (usually, sorting out the school bags or taking the register) - anything but being let loose into a sea of children. Some children do best if they get to school a little early, and are the first into the classroom, again to avoid that feeling of being swamped.
The child may need a comfort object to help them through the day. I often write a tiny note - just something like 'Mummy loves you' - which dd can tuck into her pocket for the day. Or we take in a special object - a toy - which the teacher keeps on her desk. She tells dd that if ever she feels sad or has big feelings, she can come straight up and cuddle the toy. (Whatever you do, don't copy the teacher at our school who took to using the comfort object to punish - ie. if you're naughty I won't let you look at the photo of your mum.)
Be careful of inadvertently exposing the child by doing things like asking the class to bring in baby photos (we've just had this).
Think through discipline strategies - many adopted children will respond very badly to common discipline techniques like time out and boards of shame. Of course, you have to bear in mind that you can't appear to treat one child differently from the others, so will require thoughtful planning!
Attachment: my dd has to feel there is someone in the room with whom she has a strong personal attachment in order to be able to cope. This is the only way she manages school (at home she will not be left on her own at all - not in bed, not on the toilet, nowhere). This means that she would spend the day wrapped round the teacher if she could, which is obviously far from ideal for the teacher. At the worst times, dd's teacher has coped by letting her follow her round the room with one hand on her shoulder (better than wrapped round her legs, but still not great). It's difficult for all and requires a lot of home-school communication.
dd also invests a lot of her emotional safety in her friends, and this can lead to lots of intensity and anger when things go wrong.
Keep an eye out for nasty comments from the other children - or actually even innocent comments that can be very hurtful for an adopted child. Natural curiosity may lead to, "Why didn't your mum want you?", or "But who's your real mum?" which can be difficult to manage. I don't know the age of your class, but I think a proactive approach which sets standards for manners and respecting all kinds of families and backgrounds is always a good idea.
And lastly (because I've gone on too long) don't forget how important PPP can be. For example, my dd's PPP is used to pay for in-school therapy, which is a useful source of advice and support for her form teacher.