making a school with plastic bottles

An elementary school in Guatemala has made a school with 6000 plastic bottles.
Students cooperated together and helped build their school. They took part in making the walls and collecting pet bottles.
But, plastic bottles are too weak to use for building..don't you think?
Therefore they came up with an idea that they first make a fence filled with plastic bottles first.
Then, they covered the fence with three layers of cement. They say that, by doing so, they could make stronger walls, and the strength is almst equal to the normal one.
The school with pet bottles looked nice, and you can't tell it'sfrom the outside lookthat it used plastic bottles for its building.
I think the idea is interesting, but still it can be dangerous because pet bottles are weak and easily to get old.