China runs at 1,742 hours in school with 1,374 instructional hours per a year of 221 days.
Korea runs 1,442 hours in school with 1,067 instructional hours per a year of 225 days.
US runs 1,303 hours in school with 1,061 instructional hours per a year of 180 days.
Japan runs 1,593 hours in school with 1,057 instructional hours per a year 223 days.
Canada runs 1,358 hours in school with 979 instructional hours per a year 188 days.
Englad runs 1,271 hours in school with 953 instructional hours per a year 190 days.
I find those are the ones that have the most merit off the charts. So by those stats, the US has one of the top three instructional hours, and the least amount of days in school. I think extending the school year would do a better job then extending the time in school. Well or extending the school year with leaving the same length school days. If you look at it, the countries with the longer school years are putting a better output then the US. I truly don't think lengthening the day is going to do much good, though maybe sticking a mandatory 30 minute lunch and having schools run classes at 55 minutes a day, with class changing breaks to fit the time needed.
Also another thing we really need to do is get parents more active in their kids education, most parents tend to be to busy with something else all the time in this society to even care. Though another option they kid do is add after school tutoring for classes available for students who need it, that could help right there if we had enough students to give time to do so. I know a lot of students that after school tutoring ended up helping them perform better, though they ended up having to depend on their parents to pay for it. So I think this would give the students the option, instead of forcing all the students to have to sit longer in school.
EDIT: Also another thing they could look at is removing the cap for between 14 and 21 days of absence meaning automatic fail of all classes. That system killed me a couple years due to catching the flu, and since I didn't go to a doctor but private medical I ended up failing since a doctor that isn't from a hospital can't hand you a note to excuse you of that cap. I had all my days but a couple excused, just didn't have doctor's notes or when I did they weren't good enough since I went to my friend's dad that was a doctor of his own private clinic. I would be a A and B student until I hit that point, I would still keep the grades, but they would turn straight to no credit on my cards. I don't see how it would be a problem if a kid missed that many days if he actually put effort into school. But instead they would rather run a daycare system instead since most kids at that point only show up to avoid problems with the law. I knew about 40 other people in my school that were at that point. (A couple had successful appeals to the school board, others got ignored and/or just thrown out. So the ones with successful appeals ended up still getting their grades and credits.)
Sources:
www.timeandlearning .org/resources/International%20Data.ppt (pulled my stats from this due to being a bit more recent.)
http://nces.ed.gov/pubs/eiip/eiipid24.asp (Old but still goes along with the stats.)
http://www.ascd.org/publications/researchbrief/v3n10/toc.aspx#rb_v3n10_study (didn't grab these since they didn't use an average time.)
http://www.oecd.org/dataoecd/41/14/43633767.pdf (not totally on topic but a support argument type document.)
http://educationalissues.suite101.com/article.cfm/is_more_time_in_school_better (This source says the US spend 905 instructional hours, but the source they got that from doesn't exist so I don't know how true that is.)