A fourth grader who brought a small amount of cocaine to school and shared it with his classmates at a South-Central Los Angeles school was returned to the custody of his mother after a closed court hearing. The youngster and his 16-year-old sister had been placed in the protective custody of their grandmother after the Oct. 6 incident at Martin Luther King Jr. Elementary School which sent two other children to the hospital. The boy's mother, Wilhemina Jackson, 35, was charged with child endangering and maintaining a house where narcotics are sold, but the charges were later dismissed. Rory Heidelberg, the man who allegedly supplied the cocaine, is to be arraigned on Jan. 29 on a charge of inducing a minor to violate the Health and Safety Code.