A longer school day should allow for more "play" time to be built in which then could help with the attention span issue. Hell, when I was in elementary school, we had a 15-20 minute recess in the morning, another one after lunch, and a 10 minute recess circa 1:45 pm. When my kids hit elementary school, they only have a 15 minute recess at the end of lunch. This bothers me a little bit. So if the result is a less harried pace which then has more "play" time for the kids (like when I was a youngin) then I think a slightly longer school day is a good idea.
A longer school day, for me at least, is only relevant to the elementary and middle schools, but moreso with elementary school. Hence none of the issues that you mentioned for high school students are relevant. (And I don't mean that to diminish the issues you raised, just that for me I wouldn't consider lengthening the school day for high school students.) What could be done to help hs students is starting the school day later than what many districts currently do. Classes beginning at 7:30 am is ridiculously early for a hs student.