With a ~$400 budget, I say +1 for that Asus T100 tablet + keyboard/battery combo.
It's a functionality champion. If you think storage is a problem, you can get a 64GB microSD for cheap now (~$50?) or even a 128GB one. Furthermore, the keyboard has a USB 3.0 port so she can always carry an external 2.5" drive to keep the larger files like movies or whatever.
I think most teens nowadays just consume everything directly from the web (listen music from youtube channels, watch movies/series from netflix, etc.) and are less dependant on large storages like we grew up to.
Tablet mode will definitely beat chromebooks when it's used to read school material in PDFs.
Battery life with the keyboard dock on the Asus Transformer line is brutal. She'll get >12 hours easily with the added battery of the keyboard. She should be able to use it unplugged for two days, as Windows 8.1's connected standby seems to work quite well with BayTrail CPUs.
There's a version of the T100 with a 500GB HDD drive in the keyboard dock, but it should be a bit more expensive.
P.S.: Don't go for a bulky and thick laptop (whatever the screen size) and TN screen. Kids hate those.
Unless she's a gamer and is willing to sacrifice aesthetics for a discrete graphics card.