"She is student" isn't grammatically correct in English. We need to use an article when we talk about professions. In Spanish, there is no such grammar rule. It's normal to omit articles when talking about what someone does for a living. (Ella es estudiante, él es abogado, etc)
It's not natural. Use "Ella es una estudiante", as it's the way Spanish (and Romance languages in general) expresses professions or occupations.
Only use the article when you add an adjective: Estar un estudiante inteligente.
You will get use to about a while.
Being a student, or an engineer has the same grammar.
You won't use the indefinite articles with professions or occupations in Spanish.
The sentence including the indefinite article looks typically English, and looks like an English speaker trying to talk in Spanish using English grammar. So, just use Subject + Verb + occupation/job.
When you add an adjective, you will use the article:
Ella es una estudiante muy inteligente.
I think in Spanish we have the 2 forms. One speaks of attitude, for example: "él es estudiante" although we might prefer to use "él es estudioso"; there is no implication of attending any institution. The second form, is to distinguish or indicate the status of the person in a teaching/education institution, for example "ella es una profesora y él es un estudiante". So, I think DuoLingo should allow both answers as correct.
It's not the common way to express jobs/occupations in Spanish.
When we talk about occupation or something that categorises people, we use "Es". Examples:
"She is a girl" would be: Ella es una niña (Even though she'll eventualy be una mujer)
"He is a teacher" would be: Él es maestro (Even though he'll retire)
I know Spanish verb conjugation is hard, but we can do it!
Read the very good answer above your question.
Estar is for moods, or states (for instance, states of health)
Ser if for things that are considered as a part of the nature.
For instance being a man/being clever, and it's also used with professions/occupations, (like being a part of yourself).
Because both are accepted and correct pronunciations. It's a regional thing.
"Ay-ya", with the "ll" pronounced like "y" in Spain for instance.
And "ll" pronounced like an English "j" in some other places.
Ser vs Estar (in this case, someone asked about es vs está) is actually not a matter of permanence, however it is often taught that way. There are many exceptions to this rule, because the real way to tell if you should use Ser or estar is whether the aspect of something you are describing is a defining aspect of that thing, or if it could be removed to maintain its identity. For instance “Él es cubano” because if, I’m fact, the man was not Cuban, he would be a totally different person, “cubano” is a defining essence of this man. However “Usted está muy feliz” because whether or not you are happy has no effect on whether or not you are YOU. Here’s a website I found helpful that explained it a little better: https://therootofborscht.wordpress.com/ser-or-estar/
I struggled with Spanish because they taught me this stupid rule (permanent vs impermanent). And I was always wrong when doing Spanish exercises with ser/estar.
The day I understood it was about the essence, the nature vs the state, I stopped to make errors.
Also, we have to remember that "ser" is always for jobs/occupations as they are considered "essence", "nature", like being a man/a woman.
And "estar" is always for locations (permanent or not), because the location is not part of what you are.
It's not stupid, it's called grammar. English and Spanish grammar are different, because if they were the same, it would be the same language.
English grammar wants you to use the indefinite article, and the Spanish grammar wants you to not use it (when talking about professions/occupations).