Regarding the « allocation de rentrée scolaire » :
If your child goes to school in France, is between 6 and 18 and if you earn less than a certain amount (for more details, see :http://www.caf.fr/wps/portal/particuliers/catalogue/metropole/ars ) you might receive « allocation de rentrée scolaire ».
It's an amount of money given to some families to help them with the costs of « rentrée scolaire ».
If your child is between 6 and 15 and if you are « allocataire », you have nothing to do, your CAF will do everything for you.
But, as soon as your child is 16, you must send to your CAF a « déclaration de situation » (you have nothing to do to get this form, your CAF will send it).
Why ? Only because in France school is compulsory until 16. So, if your child still goes to school after he's 16, you can receive this money. If not, then you won't receive it.
Basically, what your CAF will ask for is a « attestation scolaire » or « certificat de scolarité ». A document stating your child still goes to school. This document will be given to you by his school. If not, just ask them.