Ah, yeah, good question. It sounds OK to my :ear: to just say "you and each student/co-teacher will need to create *a* personal account" instead of using "your/their". It's clear from context that the ratio is one account to one person; I don't think there's much danger of people interpreting "a personal account" as "one account for everyone to share". 

Suggestion for the second part: "if you or they don't have one already". It's slightly clunky, but I think it reads a little smoother than "you/they".