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.
Sounds definitively like coming back to work but actually performing at a slower pace until one gets up to speed. In Spain, many times after say a 3-day holiday, the first day scheduled back to work is taken by many as a day off and called the "resaca". Resaca in this manner is a "hangover day". You need that one more extra day of vacation, at home, to get ready to go back to work, whether you have been drinking or not. It's a simple difference between working and actually doing work. Don't try to understand the concept nor poke holes in it, just accept it for the laziness it represents.