1) kids have more homework than ever. Longer school day = more classes so more homework with less time to do it in
2) extracurriculars are extremely important to the college application process. Longer school day = less time and less willingness for extracurriculars
3) teachers are overworked and underpaid as it is, and usually stay long hours after school anyway. This would mean they would have to stay even longer hours for grading/maintenance/etc, leading to more exhausted teachers
4) the school day is already mostly fluff. More fluff is not going to help anyone
5) College-bound kids already have enough trouble adjusting to the freedom of college once they get there. A longer school day would mean more hand-holding and an even more difficult transition
And I'm sure there are many, many more.