As a high school student I can say with certainty that a schedule change would be bad for high school students. I know that it isn't all about the high school students in our district, however, I don't think that weather is a good enough reason to change the whole schedule and make school that much harder for those in high school.
Having unbalanced semesters, as in Calender A, isn't a great option for us. It gives teachers a hard schedule to work on, since they have less time in the first semester than the second semester. Calenders B and C both propose having finals after break. How is this beneficial to us? It's not. Many students are away over break, or busy doing other things, not studying. I like having a break after first semester to relax and get ready again for second semester, and I think that many other students do to. Winter Break wouldn't really be a break if there were finals after it. Also, having that long of a break before finals would lower grades.
Having a start after labor day may have the advantage that weather is better and schools won't get overheated, but it shortens class time for AP and IB classes. AP testing happens in May, and taking many AP classes myself, I know that AP students can use as much class time as they can get. If we start later, that's two or three more weeks that we won't have for learning.
I believe that the schedule should stay the same, because right now it works pretty well for high school students. Many middle school and elementary school students probably don't care if their schedule is changed. Although the schools are warm during August, it is probable that they will got just as hot during June, if school were to go that late.