The 9th, 8th and 6th arrondissements are the best ones to look at.
The liveliest and best areas to live in Marseilles city centre are Préfecture, Castellane, Perier, and Cours Julien.
The school is located in the 9th arrondissement, in the Luminy area. As a general rule, it is advisable to choose somewhere that is close to a Bus 21 stop which goes straight to the school.
You can check public transport in Marseilles at: http://www.rtm.fr/