Translation:The teacher spoke with the students.
I wrote: "The teacher spoke with his students," and it wasn't accepted. Shouldn't 'his' be accepted here?
Spanish often replaces mis, tus, or sus with los/las. For example - "Creo que piensas con los pies," means, "I think that you think with your feet." Not the feet, but your feet.
Am I missing a rule regarding when an ownership pronoun can be replaced?
The possessive forms are not used if the possession is inalienable, i.e. there is no doubt that the object belongs to that person. Most commonly it's about body parts and clothes that they currently wear. Students have a bit more freedom in that regard and they don't have to belong to that teacher.