Gosh, are you me ?
It is so tricky nursing an older child and well meaning as other people advice is, unless you have done it, I don't think they can truly understand.
I am nursing my children. My oldest is 4.3 year old and I have been thinking she will wean for the last couple of years.
I have tried talking to her, 'don't offer, don't refuse' (didn't work she just asked more) and saying 'no' (ending up back tracking as she would be upset, get ill etc)
Like your child, she happily goes to grandparents for a couple of days. Once she was away for 4 days and when she returned home wanted milk straight away.
When DH puts her to bed, she will either request milk when I go upstairs to the bathroom or ask for a milk feed before she goes upstairs with DH.
Some days she forgets and we can go 2 or 3 days without a feed, but as she sees me feed the younger two, I guess that reminds her.
Of course when she is ill, she nurses as much as she needs. I am grateful that she was still breastfeeding over the last couple of months when we have suffered several D&V bugs and she weathered the illness much better and quicker than her non-breastfed school friends.
Please try to worry about the school element, it is very unlikely to be a problem. My reception attending DD1 does not talk about or mention nursing at school. From talking to her, she just thinks this is something every child does, like brushing her teeth and having a bath etc. When I floated the idea with her that the other children won't be nursing, she was shocked and said that they must be sad that they don't get milk anymore...bless :)
Bottom line for me, nursing is one way to forge a bonding link between my children and me, alongside other bonding activities (cuddles, kisses, spending time together doing stuff etc). I would need a very good reason to try and prematurally break this bond by weaning before she is ready, without a good reason.
I know in my hearts of hearts that she will wean, when she is ready. But as she gets older I have doubts, wondering if I should be trying to more actively wean or not.
I am planning a visit to my local LLL group soon, as they have cheered me up in the past. I find it very to useful to be able to chat with other like minded mums, who understand the issues of nursing an older child.
Also highly recommend reading Ann Sinnott's excellent book called Nursing Older Children. I found it very helpful and interesting reading.
HTH