Subject: Why do the planets orbit in the same plane?

Why do all models of orbits always use a flat disc showing all the 
planets revolving around the sun in the same plane. This seems to be 
a little earth-centric, i.e. two-dimensional view of space. Since 
space has as much "up" as "breadth" and "width", the planets could 
orbit the sun in orbits that are in differing planes. The same goes 
for all models of gas clouds slowly forming the planets.

I hope to make the answer to this question the central part of my 
school project!

Re: Why do the planets orbit in the same plane?