After his mother, not his real one she was dead, had found out about his change in psychological standing, she gave him back his wand. With the return of his wand he decided he could embrace his wizarding life again. Sherlock was shocked when John came down the stairs in a Gryffindor scarf. It was even more of a shock for John to see Sherlock wearing a Slytherin scarf.
"Who would've thought that," John said.
"Indeed," Sherlock said. "Year below me I believe…" Sherlock had gone on to muse about many other things before coming back and saying. "I would've thought you'd be a Hufflepuff…"
"Yes well I would've thought you were a Ravenclaw..." John had countered.
"The sorting hat had said the same thing until he found something, or rather couldn't find something, in my head that he thought a Ravenclaw would know."
"Oh, what's that?" John said.
"The Solar System," Sherlock said. "And Mycroft was in Ravenclaw."
"Wait," John said, ignoring Sherlock's attempted subject change. "You actually don't know about the Solar System?"
"No, John," Sherlock replied. "I find the information unnecessary, useless."
"But its Primary School stuff!" John countered.
"There are no Primary Schools in the Wizarding world John," Sherlock said.
"What…?" John said. "Oh right, you must be pure blood… My Dad was confused about the concept of Primary School."