She is maintaining a public persona, and

 

she feels she is doing it for your sake. So she does not have to be nice to you because you owe her since she is doing such a good job at it.

The way she is at home is her true self. Your doing things for her at home is seen as for the purpose of upholding that same public persona.

Maybe since she was younger, the emphasis has been on being a good student, rather than a good person.

Hope it's just a phase. Remind her to see the intrinsic value of what she does for others at school. Show your love for her for who she is, not what she does outside.