Tennessee school systems posted their highest overall graduation rate - 88.5 percent - since a 2011 assessment system went into effect, the state's education commissioner announced Tuesday.

The 2015-16 rate is up almost a full percentage point from the prior year, according to state figures.

Tennessee switched to a state report card grading system in 2011 designed to impose a more demanding standard for school systems. Education authorities have made it a mission to boost high school graduation rates in Tennessee.

*State figures show 12 school districts improved by at least 5 percentage points.

*According to the state, 95 districts posted graduation rates that were at or above 90 percent.

*A half-dozen, including Alcoa City, had rates at or above 99 percent, the latest figures show.

Knox County's rate for 2014-15 - the prior year - reached 90 percent.