He is almost surely a domestic shorthair. As most cats are, its just a very few percent whom belong to a breed,
A hint of a back door open?? Nay, not really. If he were a purebred you would know this for sure. Because, without papers, or at least solid evidence, he will always be counted as a domestic. Very few breeds are SO typical, the looks tells for themselves.
The color is black, and that white medaljong on the chest is usually called for a locket.
Could you please tell more, what is this school project about? We could perhaps put in some ideas or remarks.