From one of your links
The average salary for public school teachers in 2011–12 was $56,643 in current dollars (i.e., dollars that are not adjusted for inflation). In constant (i.e., inflation-adjusted) dollars, the average salary was about 1 percent higher in 2011–12 than in 1990–91.
First that average, I expect, represents a few quite high salaries and quite a many lower salaries. No teachers I know even hit that average, even after teaching a few years in a district that really gives a damn.
Note, your other link claims that $6800 is for "instruction," not "teachers." And it says:
Instruction expenditures include salaries and benefits of teachers and teaching assistants as well as costs for instructional materials and instructional services provided under contract
Note that your links disagree on total expenditure per student.
In any event, that seems like low pay for such important work.