CHANDLER, Ariz. -- Just hours before Sam Valdivia, 18, was stabbed to death, his high school math teacher called his house to wake him up.

"It was 11:45 at night, but she told my mom that it had to do with school," said Valdivia's older brother, Julio.

Sam Valdivia took the call, and minutes later he snuck off to the house of his teacher, Tamara Hofmann, 48.

All the while, Hofmann's boyfriend and former student, Sixto Balbuena was driving from California to Hofmann's house.

Balbuena told police that he entered the house sometime after 2 a.m. and found Hoffman and Valdivia naked in the bathroom.

Police reports allege that Balbuena punched and kicked Valdivia, before stabbing him in the stomach with a kitchen knife.

He died later at the hospital, and now his family thinks that Hoffman was just using him to make Balbuena jealous.

"Why else would she have called him at that hour," said Julio Valdivia.

Chandler police would not confirm this theory, only saying that their investigation is focused on several different things, including whether or not Hofmann dated other students in the past.

Hofmann is currently teaching at El Dorado High School.

Principal Ramona Gonzales said she suspended Hofmann after the incident, and she's talking with students to find out more about her relationship with Valdivia.

Nevertheless, Gonzales defended her hiring of Hofmann.

"She had 14 years experience, and letters of recommendation," said Gonzales.

Balbuena is in jail in lieu of a $100,000 bond.

Meanwhile, the Valdivia family has set up an account to help pay for funeral expenses. Anyone interested can donate at Chase Bank locations under Valdivia's mother's name, Placida Maldonado.