Pricing Rule - 1 teacher free with each 10 students

Folks,

Does anyone have a pricing rule that would help me with this?  Scenario is that a teacher books 14 students at the Student Price Type and 2 teachers at the Teacher Price type.  What I'd like the rule to do is drop one of the teacher tickets to be $0.00.  I've created a rule (see the screen grab) of the type Multiple Price Type which I thought would do the trick but it doesn't appear to work.

Can anyone point out what I'm doing wrong?  Thanks in advance for any advice!!!

Martin