By Rami Ayyub (Reuters) – The Michigan teenager charged with first-degree murder in the deadliest U.S. school shooting of 2021 will assert an insanity defense, his lawyers wrote in a court notice filed Thursday. Prosecutors say they intend to rebut the defense by Ethan Crumbley, a 15-year-old sophomore at a high school in Oxford, Michigan, who is charged with first-degree murder in the Nov. 30 shooting spree that killed four students. Crumbley is being held without bond in an adult jail in Oakland County, although isolated from other inmates. Six other students and a teacher were also wounded …