I don't really recommend use "Potunghua" to teach Chinese.
I've seen some kids they really failed on Chinese language. (Parents are keen on "Potunghua" for Chinese subject.)
At home, we already used two language (English and Cantonese). We didn't have any experience to teach our kids or practice at home.
Moreover, most of the subjects still teaching by English in Secondary schools today.
The public examination so far in Hong Kong is using "Cantonese Chinese".
I prefer so far "Potunghua" is a kind of side language like French or Spanish in the school. It's make our student more easy to learn other language without pressure.
This is one of the reason that I prefer other school.
During this summer holiday, try to spell some words that she knew in the school.
Checked with some relative's K3 exercises and prepare all P.1 English books earlier in August.