#it's also why Tsu'tey is so bitter and angry towards the rda/humans/dreamwalkers

[nattikay]

If you want the full story: they explain what happened in the extended edition of the movie!

Neytiri's older sister and fellow student, Sylwanin, grew increasingly angry at the RDA for the damage they were doing to the forest, to the point that she eventually stopped attending the school. One day, she and a few other young hunters decided to retaliate against the RDA by setting a bulldozer on fire. RDA troopers fought back and Sylwanin and her friends fled to the schoolhouse, possibly hoping Grace could protect them. The RDA troopers pursued them to the school and shot and killed them all, including Sylwanin, right in front of Neytiri and Grace. Grace managed to get most of the other students out safely, but.....yeah.

That's why Grace isn't allowed in the village until Jake talks Mo'at into letting her back in; she was kicked out after the schoolhouse incident (which iirc happened approximately two years before Jake arrived on Pandora). It's also why Neytiri was so keen to shoot Jake on sight the first time she saw him wandering through the forest—the RDA murdered her sister right in front of her and she wanted nothing more to do with them.

WHY DOES NO ONE TALK ABOUT THE SCENE WITH THE SCHOOL

Where Jake first goes out into the forrest with Grace and Norm, and they go to the old school . It's COVERED in bullet holes, and she refuses to tell him what happened.