What is the ruling on eating in the school restaurant in France, limiting oneself to fish, while being unaware of the ingredients of some of the salads and desserts provided? And is this considered a sin?

It is impermissible to consume meat if it does not meet the Sharia requirements for slaughter. Fish is permissible because it does not require ritual slaughter. Salads and sweets are originally permissible, unless it becomes clear that they contain impure components such as pork or alcohol, in which case they become forbidden. The default ruling for things is permissibility, and certainty is not removed by doubt.