...spending time talking to the stranger because they might just possibly become a student while I ignore the existing student because I've already got their business and it doesn't matter if I fob them off - is wrong. WRONG.
Commercially WRONG or morally WRONG? Or indeed is there even a difference? As there is a commercial arrangement with the existing customer, does that imply a moral obligation to be nice to them, rather than anyone else?
One is obliged to put first the interests of those with whom one has a commercial relationship?