Police said emergency responders found the 15-year-old victim knocked unconscious and bleeding from the face Tuesday inside Canyon High School in New Braunfels.
According to ABC affiliate KVUE, police say the victim was punched twice by another teen as he exited a classroom. The victim reportedly did not have a chance to defend himself.
The school was temporarily placed on lockdown as a result of the incident.
Another 15-year-old student is charged with aggravated assault in the attack, police say. That student is currently being detained at the Guadalupe County Juvenile Detention Center. According to KVUE, the charge may be upgraded to murder.
CBS News' Crimesider is not reporting the victim or the suspect's name because they are minors.
Julie Jerome with Comal Independent School District said counselors have been brought in to the high school to speak with students and staff who may have witnessed the fight.
The school district superintendent, Andrew Kim, called the teen's death "shocking" and a "tragic event," according to KHOU.
The incident remains under investigation by the New Braunfels Police Department Criminal Investigations Unit.