A lawsuit that could result in drastic changes to how Pennsylvania funds public education goes to trial Friday in a Harrisburg courtroom, seven years after a handful of the state’s districts first went to court to challenge a system they consider unfair. The case centers on spending disparities among Pennsylvania’s 500 districts and the comparatively low percentage of K-12 education that is paid for by state government — the plaintiffs say it is about 38%, compared to 47% nationally. Spending figures and test scores are among an array of statistics and studies expected as evidence. The trial could last well into January.