A Florida college student confessed to fatally stabbing his mother, a beloved teacher, to death because she "irritated" him, authorities said.

21-year-old Emmanuel Espinoza, a pre-med student at the University of Florida in Gainesville, was coming home for a family event. He had asked if he could stay with his mother, 46-year-old Elvia Espinoza, in Frostproof, about 60 miles south of Orlando, for the weekend, according to the Polk County Sheriff.

He arrived at her home at around 2 p.m. on Saturday. When she opened the door, her son started repeatedly stabbing her.

"She ran from him," Sheriff Grady Judd said in a video statement. As it happened, "she was talking to a family member on the phone."

The sheriff said he continued to stab her until she fell to the ground and died.

Emmanuel Espinoza immediately called 911 and confessed, according to Judd.

"We talked to him, and he confessed," Judd said. "He said, 'You know, I have wanted to kill my mother for many years because she got on my nerves.'"

When investigators asked him what his relationship with his mother was like, "He said, about an eight out of 10 that he really loved her, but she irritated him and he made up his mind today on his way from Gainesville that he would murder her," Judd said.

Elvia Espinoza was a second-grade teacher at Ben Hill Griffin Elementary School in Frostproof.

"Her family, friends, students, and colleagues at the school and with Polk County Public Schools are in our prayers," Judd said.

School board member Justin Sharpless said in a Facebook post: "She was a kind person who cared about her family, students, co-workers, and the Frostproof community."

Emmanuel Espinoza was arrested on first-degree murder charges. He was being held without bond at the Polk County Jail.

--TMX contributed to this report.