Teachers they don't want children to be pressured so that they're put off (they'd always rather you stop reading a book with a young child and leave it for a few days if you feel it's best, for instance, than push them and make a battle of it).
You don't need to clear it with the teacher (or declare to her bolshiely ) that you're tailoring the things sent home to a Reception child's needs and interests - that's just what you do (and what a teacher will be glad you're doing).
Re the OP: your teacher won't be expecting all parents to do exactly the same. IF she asks if you're practising the letters (and she probably won't unless your previous meeting made her concerned you might be actively avoiding anything that could help with literacy), I'd just say "Yes, we're trying to work them into little games" or something like that and she'll probably be very happy with that answer. Just knowing that you as the parent have read her list and noticed which letters are important ones to start with, and that they'll near the front of your mind so you can e.g. spot chances to talk about them, will be a good sign that he's getting some support in learning those sounds (and a lot more than some children will get). A win-win situation, everyone's happy .
The teacher might be concerned about what you do IF you make an issue of it and seem to be actively refusing to do anything that might help with later reading, but she really won't have a fixed idea in her head of you sitting down and doing 'reading homework' with him as the only way to do the practising.
You can use those letters a little bit more than others while occasionally playing I Spy, or spot words starting with those letters in particular on objects, or sing funny songs, or link sounds to favourite characters, or draw the odd letter in amongst the sand pictures, or make occasional letters as well as animals out of plasticine, or temporarily arrange some other toy into an 's' shape and comment on it - there are so many possibilities. Those are all things the teacher will be genuinely happy with as ways of you helping your child gradually to learn those letters, because they will all help him to learn (even if he doesn't realise it). If you are possibly going to keep him at school, then that would be a wise investment of very small amounts of time, and doesn't mean putting pressure on at all. (Sorry, am repeating
myself now I know!)
School doesn't mean totally giving up following a child's lead and play-based learning, or not letting a child's interests lead the way and inspire and be learnt from. It may never be as good as HE in that respect, because of the number of children being taught at once and the schedule, but it's not its diametrical opposite, where you can't take any account of the individual child, either.
By the way my eldest wasn't ready to read till well over five (lots of phonics games happened before that though) - and when reading did happen, my goodness it was fast! Something really does just 'click' when they're ready, and they're away - but the earlier stuff helps even if they don't really realise what (or that) they're learning.