Is a shark considered a reptile?

No, not even close! To give you an idea (the most widely used) scientific classification hierarchy is Kingdom, Phylum, Class, Order, Family, Genus, species. Animals in different Kingdoms are the least similar with animals of the same species being the same. Sharks are from the Phylum Chondrichthyes, Reptiles the Phylum Chordata (humans also belong to the Phylum Chordata) and so are very different