As I mentioned up above, a teacher working an 8 hour day is essentially "working to code" - meaning the minimum required hours they have to contractually put in. That's generally something unions do as a job action to put pressure on a district - it means not putting in any additional time offering extra help, calling parents/guardians, grading papers, staying as advisors for clubs, writing college recommendation letters, etc. It generally sucks for all involved - the teachers have work pile up that they'll have to catch up on when the job action ends, and the students get short changed on teacher availability and having to wait much longer to get assignments graded and returned.
Assuming a 180 day school year at 8 hours isn't realistic. Also, that's 180 instructional days - which doesn't include other contractually required days when students aren't present.
When I worked out my average # of work hours as a teacher vs. my average # of work hours at a 9-5 with 2 weeks vacation and the corporate holiday calendar, teaching ended up with more overall hours. That was me, though, so ymmv - not all teachers put in the same hours. Some worked more (English teachers, to be sure) and some less (Phys Ed teachers, for example).
