The latest incident took place at Hutchinson Middle School and involves a 13-year-old girl. Taylor Welch, 22, was arrested at his home on Tuesday.
Authorities said Welch and the girl became Facebook friends in December, and then started to exchange sexually graphic photos over the Internet and through their cellphones. Welch, who has only been a substitute teacher since September, is also accused of having inappropriate personal contact with the victim after class in the classroom.
"I was shocked to find out that it was my daughter's sub, too. I had to talk to my daughter to make sure that she was not a victim, too," said parent Jessica Alfaro.
Investigators became aware of the relationship after the victim showed some the inappropriate photos to a friend, who then notified adults. Welch has been relieved of his teaching duties and is being held on $140,000 bail.
In March, Ted Nishihara, a substitute teacher at Los Coyotes Middle School, was arrested for lewd acts with a female student.
The incidents are heating up the debate on whether teachers and students should be friends on Facebook or communicate through social media at all. Some teachers maintain that social media can be a vital educational resource because it is a primary means of communication for kids.
Others are increasingly concerned about the ease that teachers can overstep their boundaries. At least 40 school districts nationwide have approved social media policies for their teachers.