The diameter of the bar might have something to do with it. Is it thicker or thinner than your bar at school? I have the same problem of being able to do more at school than at home. The reason is that my bench at home is higher off the ground than the benches at school. Because it's higher, I can't get the same leverage. It could have something to do with the width of your bench at home, also. Are you sure the bar weighs 15lbs? Most weigh at least 25.

Just some ideas...