Los Angeles police detectives Wednesday arrested an elementary school teacher for the alleged sexual abuse of 20 children and an adult, authorities said.

Robert Pimentel, 57, who taught at George de la Torre Jr. Elementary School in Wilmington, La., has been charged with eight counts of continuous sexual abuse and seven counts of lewd acts on a child. The charges cover the period between September 2011 and March 2012. Pimentel has been accused of touching the children inappropriately under and over their clothing, the Los Angeles Times reported, quoting court documents.

Los Angeles Police Department detectives launched an investigation in March after several fourth grade students complained that the teacher had inappropriately touched them, the Associated Press reported.

L.A. Unified Superintendant John Deasy said that both Pimentel and the school’s principal were removed from the campus immediately after the school district office learned about the allegations in March. The principal, who was not identified, was removed because the school district authorities were not satisfied with the way he had handled the situation.

The district officials said they had prepared a notice of termination for Pimentel and the school principal. The notice was supposed to be presented to the Board of Education in April 2012.  

However, both Pimentel and the principal had retired before any action could be taken, and they are now entitled to receive their full pensions, Deasy told the LA Times.

"Can you go back and fire someone who's already retired? No, you can't," Deasy said.

Pimentel had been working in the school since 1974.

The allegations are the latest in a series of sexual abuse cases reported in the district involving the teachers and students. 

In January 2012, two teachers at Miramonte Elementary School in the Florence-Firestone neighborhood were charged with carrying out lewd acts on 23 children who were between 6 and 10 years old. The entire teaching staff of Miramonte Elementary School was relocated following the parents’ protests over the issue.

In December, a jury awarded $6.9 million in compensation to a 10-year-old student who was molested by his teacher at Queen Anne Elementary School in the city's mid-Wilshire district.