When students enter a class, they tend to spread themselves out (provided there are enough desks to do so). in biological terms, this effect would be: ?

In biological terms, this effect would be "diffusion".

Diffusion is a spontaneous movement of particles from a territory of high focus to a zone of low fixation. (ex. tea seasoning moving from a zone of high to low fixation in heated water.)  

Diffusion predominantly happens in gaseous state or inside gas atoms and fluid molecules.(e.g. The particles of 2 gases are in steady movement and if the layer isolating them is evacuated the gases will blend due to arbitrary speeds.)

why does vagina taste like fish water (asking for a friend)