How is it that I have an iq of 140-145 but im failing a class

Your IQ score is far from a guarantee that you will do well, either in school or life. The MENSA organization's only requirement for membership is that you score in the top two percent on your IQ test. Yet by education or income, they are far below the top two percent. Of the top two percent of the scores, 17.93% have a Master's degree or equivalent, 17.56% four year graduates, only one in 11 holds a Phd. By income, about 30% earn less than 40,000, 10% earn $50-75,000, 9% earn $40-50,000 and about 0.5% earn more than $250,000.

There have always been arguments about what IQ scores mean and looking at the numbers above, one can understand why. My guess is, the difference is test scores don't say anything about effort, motivation and attitude.

http://mwm.us.mensa.org/faq/people.html