Lowering the threshold for school parcel taxes -- from the currently required two-thirds majority to a 55 percent vote -- would not necessarily encourage more school districts to try to pass them, according to a report released Monday.

The Public Policy Institute of California reported that it could not show that parcel taxes, which supplement state revenue and local property taxes, for schools would expand beyond generally wealthy school districts that have them.

Only about 10 percent of California's approximately 1,000 school districts have passed parcel taxes; more than 80 percent of those are in the Bay Area. Those districts tend to have fewer low-income and non-white students.

In examining the parcel tax measures proposed since 1995, co-authors Eric McGhee and Margaret Weston found that 89 percent would have passed, rather than the 59 percent that succeeded.

Among those that could have passed a parcel tax with a 55 percent threshold were Pleasanton and Millbrae. Jefferson Union High in Daly City twice failed before passing a parcel tax last year.

-- Sharon Noguchi, staff