Your observation is very interesting. Was the test tube containing the oil and water left open overnight? Salt, because of its high ionic characteristic, has a very high affinity for anything polar. The salt that you placed on the oil does not want to be there. The large London dispersion forces that hold oils together are not desirable for a salt to be around. Therefore, it attracts other polar substances, such as water.

What you are observing is most likely water, either from the air or residual water in the oil.

The salt and the water are like the positive and negative magnet ends seeking each other out. ANY water that is in the oil or even the air is going to try to satisfy the salt's desire for polar partners. You are virtually drying out the oil by putting salt on top of the oil. All of the water will go to the top because of the strong attraction between ionic compounds and the polarity of water.

Most oils have little air bubbles in them that could contain water, are those still present? Does this phenomenon occur when you place a non-ionic, but polar solute such as sugar on top of the oil? Does the amount of water change in different environments, ex., outside, inside, refrigerator, etc? Those are some pretty cool, simple experiments that you could do to help out your class. I hope that this helps!