Hogwarts was a large place and Sherlock didn't mingle with other houses, let alone years. In fact he barely socialised amongst other Slytherins. So he never noticed John Watson (or Bond if you knew him before he fell into a cover) his later flatmate, a half-blood wizard in Gryffindor at all. Sure he noticed that band of idiots in the year above. He had even deduced that three of the five, though very few new of there being five, were related in some way and that they weren't human. But that's another story.
Sherlock had spent all of his 7 years at Hogwarts being completely bored, as was his brother who was 3 years older than him (although he hid it better than Sherlock). Mycroft, his Ravenclaw brother, had gone on to take a job in the muggle government of Britain. Sherlock had also decided to go into the muggle world for employment, as had John but he didn't know this. He had become a Consulting Detective, the only one in the world, his primary focus was on the muggle side of things he did get some wizard cases as well.
He had gained knowledge of the muggle world and with this knowledge decided to rent a flat in muggle London. However he needed a flat share with someone to be able to get it without Mummy's help, or Mycroft's.
He told has 'friend' Mike as such the morning of one boring day.