She's a ghost. That's the twist I guess, from what I remember.

But the driver seems to notice she's there

Is the driver a magic negro?

@Joeisacoolguy said:

Personally, I think the latter, but I'm interested to hear other people's thoughts

The "latter" - I agree.
I'm thinking, as she walked thru the school for the last time, she was being ignored by the school staff - in part - because they didn't want to draw parent attention to the fact they hired a "ghost hunter". They wanted to put all that ugliness (and dead child business) behind them.

Finally, though I'm not totally convinced, outside - at the very end - we see her walking past a small group. She walks on the far side of the group; on the left side of a boy as she says "Hello." and the boy turns toward her - presumably sees her.

Remember, none of the school boys saw Tom's ghost (I think) - only the lonely asthmatic kid who died. Typically ghosts were not visible to the school boys (they had to pretend / mimic).
So, in other words, it's unlikely the kid she walked past would have seen her if she were dead.

I also think she said something like - They will always be with us - even if we don't see them.

I like to think she and soldier boy lived, married and were happy ever after.
But, I don't mind a little ambiguity with the ending (though slightly frustrating).

I was thoroughly entertained.