In French we use “la rentrée" to mean the return to work after the slack period of the summer break in France. It is used by students, workers and pretty much anyone else. Is there any English expression to convey a similar meaning?
Not really, in the sense that you use it. This is because, as StoneyB has mentioned, there is no real equivalent to the French summer break in either the UK or the USA.
A literal translation would be "re-entry", which is indeed the term used when someone is returning to the job market after a leave of absence, but this isn't generally used for coming back from vacation, and isn't used if you are returning to the job you left. It would be more common after, perhaps, taking a year off after having a child, recovering from a prolonged illness or accident, or coming out of retirement. For example:
Five years ago, I won $100 million in the lottery. I find that I have managed to spend it all, so I have to re-enter the job market.
In Australia it is called "Back to school". The new school year and the cranking up of work again after the summer holidays is from mid to late January. So many people would wish each other "Happy New Year" for the first couple of weeks - probably haven't seen each other since before Christmas.