Since you can be a premium member for as less as 1 coin, not all premium members are treated equal. Premium members with more coins get more benefits than premium members with lesser coins.
Premium membership is paid for using TeacherOn Coins on monthly basis.
Rank is your position in the list when someone searches for the subject you teach. Higher the rank, higher you are in the list - which increases your chance of being contacted by the students.
Yes. Coins you chose to use for premium membership are automatically deducted from your TeacherOn wallet on 1st of every month.