Families’ interest in their kids’ education is falling in many high-income countries as school systems struggle with plunging test scores, according to newly released results from a standardized test taken by 15-year-olds around the world. Over 760,000 teenagers in dozens of countries took the Programme for International Student Assessment last year. Results released Tuesday show average scores in math, reading and science are all at their lowest point in PISA history among highly developed economies. That includes continued declines in reading and math scores in the U.S. China, Singapore, Taiwan, Japan and South Korea scored the highest marks on the…