Well, back in like 6th grade I remember I had to do something like this, so what I did was go outside and gather a large amount of twigs and sticks all the same relative size, and I would have them all lean inward towards the center and tape/glue them together at the top, thereby forming a teepee shape.
Or you could always use the log cabin technique with walls made out of twigs, and a roof made out of some hay-like material.