To me, the biggest reason not to do this is one that is sort of implied by the third point that the first answer makes.  In other words, it is not clear to me that this change would help students.

The problem is that I think students would end up getting burned out before the end of the day and the quality of education would drop.  It would be hard for students (and probably for teachers) to maintain concentration for a whole 8 hour day.  So they would end up in class for longer amounts of time but would not necessarily learn more.

A few arguments against this idea can include the fact that the Department of Education is a federal agency, and educational decisions such as length of the school day are decided at the state level, as are most educational decisions.  Secondly, to lengthen the school day is expensive, and the states are the ones who would have to pay.  Given the recession and crashing state budgets around the country, this is hardly feasible.  Thirdly, there is little research to suggest that the primary reason for failing schools is not enough seat time for students.  I could argue that we need more resources in order to do a better job with the time we have.  Lastly, with athletics at the high school level, extending the school day also extends those athletic contests later into the evening, which takes a toll on those students who choose to participate.