The British Council assistant pages are useful. Their information booklet for FLAs is here.
In the run-up to exams the assistant can help with last minute practice and preparation. They can also be on hand on the orals day itself to do last minute practice.
The main benefit students will get from the assistant is lots of authentic language input. Next most important is the opportunity to develop oral skills within a small group.
With KS3 and KS4 students my own feeling is that the assistant is better used in the classroom with the teacher. Why? There may be behaviour issues with this age range and taking students out of lessons disrupts your teaching. At lower levels too, the regular teacher has a better grasp of what students have done and will understand.
So, with younger groups I would keep the assistant in class and use them to support your regular work. We would usually plan our lessons so that the assistant would be there to support pair work. Typically we would plan a lesson so that the teacher and assistant would model a dialogue or communication gap activity. Then, once the class was underway with pair work, both teacher and assistant would be available to listen in and support, correcting as appropriate.
At the start of the assistant's year (usually early October) we would introduce the assistant to younger classes (Y9-11) by using the sheet below. You could design a simpler one for Y8. I would not do this task with Y7. The teacher would interview the assistant with reference to the worksheet and the class would take notes. This would then lead to whole question-answer/feedback, perhaps with an extension task for an able student where they would give a mini talk about the assistant in the third person.
We would try to leave time for a few questions in French or English.
These lessons always went well. The format works so much better than the under-prepared "what questions would you like to ask" type lesson. The class got loads of good quality, finely tuned listening input, were actively taking notes and heard a new voice. Each question provides an opportunity to revise the alphabet. If the assistant started using language which was too hard, the teacher could re-word statements or lead the assistant towards easier language.There was also the opportunity for some oral work.
The assistant should ideally see the sheet beforehand. tell them not to worry too much about the absolute truth.
Here is the sheet we used (available on frenchteacher.net too):
INTERVIEW AVEC NOTRE ASSISTANT(E)
1. Il/Elle s’app____ ___________ ___________
2. Elle a ________________
3. Elle a _____ _________ qui s’appelle ____________
4. Elle h__________ à ___________ dans _____________
5. C’est une ville ________________
6. On _____ visiter _____________ et ________________
7. Comme passe-temps elle aime, par exemple, __ ____________
8. En ce moment elle travaille __________________________
9. Elle va finir _____________, puis elle ________________
10. Le weekend dernier elle a ___________________________
et elle a ________________________________________
11. Hier soir elle a mangé ______________________________
12. Et elle a _______________________________________
13. Le weekend prochain elle va _________________________
14. Comme musique ___________________________________
15. Un film qu’elle aime beaucoup, c’est “___________________”
16. Récemment elle a vu “______________________________”