How much should a 16 years old student minimum sleep at night specially before exams (to avoid trouble in memory and concentration)?

Depends on person: The answer somewhat depends on the individual but for the two nights before i'd recommend between 8-10 hours each night. You'd be better off sleeping consistently with regular sleep and wake times for 2 weeks prior to the test.

No < 9 hours: During the growing years a teen needs 9 or more hours of sleep per 24 hrs. Growth spurts and physically demanding activities can add to the need. Failure to keep up with the sleep requirement can show up in a variety of ways. Poor academic performance in school, sleep walking, etc. After growth stops & activities lessen, 8hrs may be adequate.