Both are correct.
The second shows "backshifting", the tendency to shift present tense verbs to past tense when reporting what someone said.
Backshifting is more likely when the person telling the story isn't certain if the reported fact is still true. So the first report suggests that the person making the report believes that she is still an intelligent student. This is likely if you know she is still a student.
Did you read our daughter's report card. Her teacher said that she's an intelligent student.
The second report suggests that the person making the report is less certain.
If you changed it to "hungry" you'd be more likely to backshift, because when you report the speech you don't know if she is still hungry. Similarly if you are reporting something said many years ago, you'd be more likely to backshift (since she is not a student anymore).
Her teachers said that she was an intelligent student, but it is not until she won the Nobel prize that I understood just how intelligent.