Raise flag for one user at a time

Hi there,
My problem: User A reffer user B,C and D.

I guess that 'flag' is ment for users who paid/signed up for a service, so for instance, if user B decided to pay, I should be able to raise his flag.
Currently, I can only 'flagout' the reffered user.
maybe I missed understood the flag idea, or there is a better way to accomplish that?

The final idea is to have a teacher which students could register through him, the teachers gets new email upon each registration. when one of the students decide to pay (a symbolic fee btw), the teacher see's that and get's notification mail.

you'r problably asking - who's the one who will raise those flags, well - we have people who actualyl call to those students, when someone will pay, we should raise up his/her flag. so the teacher will get a notification email and will see that in his referral table.