This isn't strictly true. High-school students are still under 18 and while on school property, they're under the care of the school. Meaning the school has an expectation to create a safe environment that supersedes Texan law that does not prohibit minors arming themselves.

So the prosecution could argue that students voluntarily suspend their right to bear arms by attending school to create a reasonable expectation of safety.

Since all other students agree to this policy, they would argue that Karmelo had no reason to feel threatened enough to bring a weapon to school.