A 'six sense' is often confused with intuition, empathy, and a strong synergy(am i using that right?) of your five primary senses. There is no sixth sense.
Intuition, and empathy combination: The feeling of 'knowing what will happen when' comes from your ability to read subtle signs of those around you. Even if you do not realize its what your doing. The 'teacher will call on me' situation people are fighting about comes from your ability to catch a glance from the teacher your way and instinctual readiness. Your not using a sixth sense, you know that the teacher will call on people names because your paying attention. Thus your body is bracing itself, getting ready. Its not that you 'know' the teacher will call on you your just getting ready for it. As someone already said people who think they always know they are going to be called on have selective memory. You feel the same way every single time, not always to the same extent, but when your anticipation is confirmed and your called on your body snaps into overdrive thus you are likely to remember those instances since they stand out more then the instances when you were not picked. As well you are more likely to begin to pick up on signs when your going to be picked which leads me to....
Recognizing patterns without recognizing patterns: Every day after school from grade 10-12 i would watch the Simpson's after school. In grade 12 i would start thinking about an episode of the Simpson's randomly, it was always the episode that was on that day when i got home. Me and my friend kept track of it and all but twice in the course of three weeks i got it right. Does that mean i am psychic? Far from it. The channel i watched played the episodes in a specific order. After two years of watching that show every day after school i knew what episode was going to play. I just didn't know I know. I saw a pattern i didn't recognize i recognized. At any point you feel you have seen the future you have more then likely began to pick up on day to day patterns. You have no future sight, your simply have begun to recognize instances that have already happened before in your life and can make good clear guesses at whats going to happen before it does. You feel like you have done this before or seen it before because you have, you just don't remember that you remember.
Sensing those around you: This is pretty much a mash up of your five primary senses, Sight, Hearing, Smell, Feeling, and Taste. Your body can detect things very easily. People can feel the disturbance of air as someone moves. People can smell the scent of a human breathing. People can hear the sound of a blade of grass snapping under a foot. People can spot the slightest changes in the shadows. People can taste a mans sweat on the wind. If trained. Even more amazing is the humans who can blend these senses together. An immaculate blending of these senses, relying on none and mastering all of them. A human can sense things around them that borders on supernatural. Though still not a sixth sense it is a unified force of all these senses.
Thank you for reading my short essay.