1) Peyton returned a while ago.. March 4th to be exact. She's not been very social though, and there's a good chance that Summer has the room to herself a lot, because Peyton keeps herself busy at the library or in the lab. But, if she needed a babysitter for Sebastian, Pey would watch him.
2) Jack should flirt with Cherry! She loves flirting, though she's trying to be good about not doing anything more than that with students ;)
3) Also, Cherry sent an email to Erin a while back. Not sure if you forgot, or if Erin simply ignored the email. For some reason, I can see them getting along. Erin might even get along with Peyton.
4) Erin is welcome to flirt with Charlie at her party. He never objects to a hot woman coming on to him.