Re: What causes wet beach sand to light up when you kick it on a dark night?

Your teacher was right that it is a type of plankton - although it could 
be either phytoplankton or bacteriplankton.  The phenomenon you are 
observing is called bioluminescence.  There has been a lot of work done 
recently on the luminescent bacteria called Vibrio (or Photobacterium, 
depending on your reference) fischerii.  It is the bacterium found in the 
light organ of a fish called the flashlight fish.  The bacteria do not 
luminesce until they reach a critical density or some artificial pressure 
is applied.  The latter is the type of phenomenon you are observing - an 
environmental stress causing the bacteria or phytoplankton to luminesce.