Make the entire movie from the perspective of a teacher who knew, and was nice to, Eric and Dylan. The audience never sees Dylan's or Eric's faces directly.
In one scene, the teacher is helping Eric with an assignment. Another scene the teacher is giving Dylan some pizza for a class pizza party. There are classic "hey stop bullying him!" scenes where the teacher fends off bullies. There's a scene where a tattletale kid snitches on Eric or Dylan for having porn or smoking cigarettes or being weird. The teacher talks to the parents of Eric or Dylan.
The day of the shooting, the teacher is in the classroom, teaching as normal. The shooting scene shows all the associated panic and horror among the students and the teacher as they learn the school is under attack. They hide. They pray. They cry. They scream. They see students running. But no gunshots are even heard. They are in a part of the school far away from danger. The shooting ends and the police show up. The dismissal of students to horrified parents is shown. The anger and outrage of some parents is shown. The teacher shows strength as they comfort some parents and patiently listen to complaints from some parents.
The end of the movie is the teacher alone at home looking through a yearbook and crying.
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I would personally cast John Bernthal or Dave Bautista as the teacher. A masculine man. To examine the bond between older males and adolescent males. To examine everyday courage.