Some 15,000 teachers are expected to rally today in Raleigh, N.C., in another educator uprising aimed at improving public schools. So many will leave their schools for the action that the state’s five largest districts have canceled classes. The teachers are demanding better pay and working conditions, and more funding for public schools struggling with a shortage of teacher assistants and counselors, outdated textbooks and old technology. Many hope to shift the Legislature by electing more pro-public education candidates.