...first manually move the eye further down in Z than it needs to go (so the top is below the top of the teeth). Then hold ctrl to snap, and now the top of the eye will snap to top of the teeth. (For all the OCD artists like me out there who must have preeeeecision!) :-)
This Modeling Bootcamp has been so incredibly helpful thus far... excellent work, Jonathan, and I look forward to finishing the rest of this course! :-)