LOS ANGELES -- A former special education teacher at John Marshall High School in Los Angeles pleaded no contest Wednesday to sex-related charges involving three students and battery of another student, City News Service reported.

Los Angeles Superior Court Commissioner Henry Hall ordered Fred David Johnson to undergo a 90-day diagnostic evaluation before sentencing, scheduled for April 29.

Johnson, 60, faces punishment ranging from probation to six years in state prison, said Sandi Gibbons of the District Attorney's Office.

Johnson pleaded no contest to one felony count of oral copulation of an incompetent person -- a 14-year-old male student, she said.

He also pleaded no contest to two misdemeanor counts of annoying or molesting a child -- the victims were 17-year-old male and female students -- along with one misdemeanor count of battery on a dependent adult involving a 19- year-old male student, according to Gibbons.

The crimes occurred between January and August of last year. Police said last summer that an aide in Johnson's class alerted them to the crimes.

Johnson had worked at Marshall High School since 1992. Before his arrest last Aug. 20, he had been reassigned to a non-school site.