Student's won't like it either because longer school days means more stress-related behavioral issues.
As for teachers, I can only speak for how this typically is handled. Usually, a teacher has a period of time each school day for planning. This planning time is critical for grading, paperwork, documentation, phone calls, making copies, ordering supplies, as well as actual lesson planning. To accommodate the need to squish more instruction hours into the student's day, all planning times are diverted to the student-free day. I know that personally, my planning time is absolutely critical to my instruction. Sometimes a lesson has to thrown out due to an emergency (shortened period, technical issue, lack of supplies, illness, etc., etc.). Sometimes things need to be done suddenly in the middle of a day (a behavior referral needs to be written while memory is still fresh, an emergency meeting needs to be held, more supplies are needed, a currently used plan needs tweeking, etc., etc.). Sometimes I just a few minutes to be away from kids and regain my composure. From every teacher I've discussed this with, it is a disastrous plan.