Subject: Why are sponges classified as animals and not as a colony of protists?

I will soon begin teaching my 7th grade students the characteristics 
of animals.  In one biology book I referenced (Biology, The Science 
of Life.  Wallace, Robert A., Sanders, Gerald P., and Ferl, Robert 
J.), it said animals show "multicellularity", "most animals have 
well-organized organ systems", "animals are able to move", "cells and 
tissues are bound together" to support animals, "animals require 
complex nutrients", and "animals are essentially diploid with gametic 
life cycles....Many species reproduce asexually."  

I understand there are exceptions to every rule, but it seems to me 
that the sponges break many of them (able to move, well-organized 
organ systems, cells and tissues are bound together for support).  How 
can I explain to 7th graders why they belong in the Animal Kingdom?

Thanks in advance.

Re: Why are sponges classified as animals and not as a colony of protists?