School was so much fun for DD6 socially that, to begin with, she was happy and stimulated and exhausted just from being there. Plus they did loads of art which she adored which made going to school worthwhile. She could count and recognise numbers up to 1000 so arranging numbers up to 30 didn't do a lot for her but it wasn't until the second half of the year that I realised she'd disengaged somewhat.
In contrast her teacher last year (it's the long holidays here in Australia) gave her extension maths with every single lesson and the joy she's gleaned from that has ensured that she's thrived in all areas. She's gone from being a competent but reluctant reader to working her way through all the Harry Potter books (reading the first three in two weeks interspersed with loads of swimming) and independently seeks out all space books at the library.

I think her first teacher thought the best way to extend her was to give her hard readers but her amazing teacher last year actually moved her down 5 levels initially saying that she knew DD6 could read and spell any Year 1 word but she could become nervous reading out loud and she wanted to work on that. Mid way through last year she "got" that too and her general nervousness about speaking in front of the class also abated. Given that the school readers were the very least of what she read at home I didn't care at all about the "number". There were interesting chapter books and maths puzzles in the class for her too.

So, convoluted takes aside, my advice would be :
1) keep things interesting at home,
2) If she's happy initially and overall stimulated at school - even if the academic side is way too easy - certainly advocate for her but allow for a social "settling" in period. DD6 started at 4 and I don't think not being taught to her potential initially either harmed or bothered her,
3) when it does start to bother her and she becomes disengaged/disillusioned etc step in and become "that mother",
4) And, my personal lesson, is that if a good teacher who recognises your DD's talents seems to want to "hold back" one area to focus on a particular aspect it may not be a bad thing. My daughter's reading has leapt from simple chapter books with pictures to Harry Potter level in six months which I'm sure is confidence related rather than a leap in actual ability.