If you do not have a long-stay visa valid as a residence permit, you will need to apply for a student residence permit (titre de séjour mention "étudiant") within two months of arriving in France. At the city's youth information centre, the Espace Infos Jeunes, the Students Welcome Desk will help you with the formalities and your relations with the Préfecture de la Loire.
7 avenue du Président-Émile-Loubet, Saint-Étienne
Do you already have a valid long-stay visa valid as a residence permit or a student residence permit? You can renew it on line in the last two months before your visa runs out. Make sure you do it in time or you may have to go back to your country of permanent residence to apply for a new visa.