It is reminded that a pass for public transport, from 14th to 16th May, will be provided at the registration desk.
From ENS-Cachan, the closest stop is "Bagneux" and is on the RER line B, (towards “Robinson” or “Massy-Palaiseau”). (See How to use the Paris Public Transportation)
As illustrated by red arrows in below, when leaving Bagneux station, turn left in avenue Pont-Royal and continue in Avenue Chateaubriand downhill till the first traffic light. At this point, you can see the ENS’ entrance.