Belton Police say Caysen Tyler Allison, 18, killed Jose Louis Ramirez Jr., 18, on Tuesday at Belton High School.
Allison ran away from the school after the stabbing but was arrested 20 minutes later, according to the Belton Police Department. He was booked in the Bell County Jail with a $1 million bond. Police have not said what led to the fight between the two teenagers.
The school was placed on lockdown following the stabbing.
"On behalf of all Beltonians, I want all students, faculty, and staff at Belton High School and Belton ISD to know that they are in our thoughts and prayers," Belton Mayor Wayne Carpenter said in a statement. "Please be in prayer for all involved."
Ramirez Jr., a senior at the school, was described on a GoFundMe page raising money for his family as a "very kind, loving and gentle young man."