Hello Romaric,
No code is saved in execution servers (jail servers).
The teacher's and student's code is saved in the Moodle server. These source codes are sent to the execution server when you run or evaluate the student code. Once the execution or evaluation ends the code and results are removed from the server. You can trust in restarting/removing/changing your execution server with no effect on the code of teachers or students.
Best regards,
Juan Carlos