To get the ENS Paris-Saclay, by the express subway (RER), you need:
20 min. from the heart of Paris and the Quartier Latin
30 to 45 min. from Orly-Airport
60 min. from Charles de Gaulle-Airport
Be sure that the sign « Bagneux » is lip up (never get down at "Arcueil-Cachan" station, you would then have to walk half an hour more !) and get down at"Bagneux" station.
When leaving the station, make a left on the main road until the first traffic light. At this point, you can see the flag at the campus' entrance.
Line 184 Porte d'Italie - L'Haÿ-les-Roses: get down at "Camille Desmoulins" station
Line 187 Porte d'Orléans - Fresnes: get down at "Camille Desmoulins" station
Line 197 Porte d'Orléans - Bourg-la-Reine: get down at RER "Bagneux" station (then walk 10 min.)
Line 162 Villejuif - Cachan - Meudon: get down at "Mairie de Cachan" station (then walk 10 min.)