Ebola spreads via heavy droplets (through the air) (spitting, sneezing, coughing etc, produce heavy-droplets. It does not (allegedly) carry- on the air. That allows non-proximity advantages and reduces its potency as a contagion.
Let me explain - In a close-proximity environment (Cinema, plane, public transport, office, classroom, etc) ONE sneeze/cough can infect EVERYONE in that area.
Also - The virus, on surfaces (away from UV-rays) can reside actively for 4+ hours.
Everyone entering (Classroom) during those 4+ hours - Is now potentially infected.
NOT saying it WILL. Just that it CAN.