"A comida é para os professores."
If all the teachers were female, and the speaker knew that, then "as professoras" would be the only correct answer I would have thought. Otherwise I suspect it would be like saying "the food is for the gentlemen" when there was a group of women at the table. Unfortunately, I'm not a native speaker so this is just conjecture. Whether Duolingo accepts that sentence I don't know - it should.
When there is a group of men and women, you should always go for the masculine plural. Even if there are 9 women and only 1 man, you should say "os professores". Only if all of them are women, you should say "as professoras".
This rule does not apply if you are a female governor or president, and you are member of a populist party, like Dilma Rousseff here in Brazil or Pauline Marois in Québec/Canada. They love to make it clear they are talking to women and men: "brasileiras e brasileiros", "québécoises et québecois". Although these are different languages (portuguese and french), I think they "studied the same school", if you know what I mean.
Thank you. It's great to have a native speaker perspective. Can you clear up my small doubt about the group of teachers sitting around the dining table. If someone who can see that the group of teachers is all-female says "A comida é para os professores", do the women feel slightly offended or is the bias towards masculine words so ingrained that they don't bat an eyelid?