Is it considered tale-bearing to inform the teacher about a female student who cheated with her classmate on the exam?

As long as you were unaware, there is no sin upon you for informing the student about what she asked, due to the hadith: "Verily, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are forced to do." However, it would have been more appropriate and better to conceal the student's secret, advise her, and guide her privately, rather than informing the teacher.