Texas Department of Public Safety told ABC News two people, including one student aboard the school bus, were killed.
The Austin-Travis County Emergency Medical Service said the crash happened at 2:17 p.m. on State Highway 21 near the Travis/Bastrop County line, about 50 miles east of downtown Austin.
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The EMS posted about the crash on X, formerly known as Twitter, saying crews responded to a crash involving at least 40 people.
In a news release, Hays Consolidated Independent School District confirmed that 44 students and 11 adults were on the school bus during the major crash.
The district said the bus was carrying pre-kindergarten students from Tom Green Elementary School returning from a field trip to the Bastrop Zoo.
"Parents of students on the bus have been notified directly from the campus. The district is in the process of conducting reunification. Hays CISD transportation staff, counselors, safety and security personnel, and other administrators have all joined first responders either at the accident scene or at the reunification location," the district said.
Texas DPS told ABC News that the "very serious accident" involved the school bus, a Dodge Charger with passengers, and a concrete truck.
Officials said helicopters took four patients to St. David's South Austin Medical Center, and three others were driven to the hospital.
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DPS said a man driving the Dodge Charger and a child from the school bus died.
Details about the crash's cause and the victims' ages or conditions were not immediately provided.
Bastrop County is included in the Austin-Round Rock, Texas, metropolitan statistical area.
This is a breaking news story. Check back with this article for updates.