Also, studies have proven the 8 hour sleep ideal to be a myth, and that rather humans have forced these patterns. What's optimal is sleeping 'x' or whatever hours at night, and having a power nap straight after lunch. This obviously isn't possible though with school and work.
So really, it all depends on the individual and at how many hours sleep you are okay with
From personal experience, I'd advise atleast sleeping minimum 6. Ideal amount is 8-10 hours a day. ^^ As this user said, there is no correct or scientifically proven amount.
During exam period, you should be sleeping more to rest your mind for maximum performance.
I usually sleep for 8 and a half hours during the school week. Then again, I get driven to school. If I had to catch public transport to school it would be far less.