The school year for teachers runs from August 10-May 31, so you're a bit off on the 70% estimate. Teachers work 50+ hour weeks during the school year and still work during summer. I have 165 students. Every day I have to prepare lessons and assessments, teach classes, grade all the assessments and record everything in the grade book. Then there are disciplinary issues and phone calls home to parents, meetings with counselors and special educators, individual education plans and 504 plans, performing mandatory "volunteer" work after school hours, and sponsoring a club (required in DougCo). 

Even if it only takes me 3 minutes to grade each student's work: 165 students x 3 minutes each = 495 minutes. That's 8 hours and 25 minutes of grading at least once or twice per week.

In summer, teachers attend professional development and are required to take classes to maintain our proficiency and earn relicensure credits. There's also CPR, first aid, mandatory child abuse reporting, etc., etc.