In grade school. They borrow a little bit of the Tomo and Yomi backstory as an homage of sorts. They've been in the same classes together UNTIL high school, though. That's the major difference. Sachiko was the one to approach Miko because she was different. She couldn't explain how or why I mean she was just a kid, but she wanted to make friends. And Miko, a super shy and troubled girl, decided to give it a shot. [there are lore reasons behind Sachiko's "she's different" reasoning, but I can't say, even in this era of the reboot-making hiatus!]