Parents who want to lift the cap on charter schools in Massachusetts are taking their case to the State House. More than 500 people are expected at a rally Wednesday to urge Governor Deval Patrick and state lawmakers to allow more charter schools. The state has 61 charter schools. Advocates say the schools do a better job of teaching children and engaging parents, and offer a necessary alternative to failing schools. Teacher unions oppose the schools. Patrick has long opposed lifting a cap on the number of charter schools, though this year, he has proposed lifting the cap in underperforming districts.