From RER B to Massy-Palaiseau/Lozere:
Take any train going in the direction Saint-Remy-les-Chevreuses and get off at either Massy-Palaiseau or Lozere.
Be careful since not every train that goes to the end-point St Remy-LaChevreuse stops at Lozere or Massy-Palaiseau.
Please make sure to check the overhead board in your RER station to confirm the stations.
Then there are two options:
(recommended) get off at Massy-Palaiseau and take the bus TransEssone number 91-06 to get off at stop "Les laboratoires" (number 2).
Indications with pictures for coming to Ecole Polytechique from Denfert-Rochereau (RER B+Bus 91.06)
- get off at Lozere and follow a path with stairs going uphill to the Ecole Polytechnique.
You will find signs easily. This takes about 15 minutes but beware it can be tiring if you carry luggage with you.