A la rentrée, literally “until the return”, means “See you in September”. In France, August is the month when the administration, schools, and businesses are closed and when families are on vacation! Many people know they won’t see each other for a while and for this reason use the expression A la rentrée!
In the expression, there is the noun la rentrée which can be translated by “back to school”. However, the expression is not only used for school, as the entire country returns to normal and people are getting back to real life!