Betty Buckley (Carrie) Carrie

Buckley played the only adult who sees Carrie as a person

Betty Buckley's Miss Collins is the film's emotional counterweight -- the gym teacher who breaks up the shower incident, punishes the bullies, and becomes the only adult in Carrie's life who treats her as a human being rather than a problem. Collins sits Carrie in front of a mirror and tells her she is pretty. She shares a story about her own prom. She tells Carrie "You'll never forget it." Then she dies calling Carrie's name, crushed under a falling beam in the destruction Carrie causes. (wikipedia)

Collins is the film's most honest character about the impulse to blame the victim

What makes Buckley's performance distinctive is a single confession. In the principal's office after the shower incident, Collins admits to Morton: "The whole thing just made me wanna take her and shake her, too." The line is the film's most honest moment about how Carrie's helplessness provokes even those who want to help. Collins recognizes the impulse in herself and is ashamed of it, which is what separates her from every other adult in the film.

The confession runs through the rest of the Collins arc. Her protectiveness in the detention scene, her encouragement in the mirror, her prom memory -- all are inflected by the knowledge that Collins once felt the same contempt the girls acted on. She overcame it. The other adults did not. Morton cannot even remember Carrie's name.

Buckley went on to define the role of Grizabella in Cats

After Carrie, Buckley built a distinguished career on Broadway, originating the role of Grizabella in the original Broadway production of Andrew Lloyd Webber's Cats (1982) and winning the Tony Award for Best Actress in a Musical. Her rendition of "Memory" became one of the most recognizable performances in Broadway history. The stage career overshadowed her film work, but Carrie remained her most significant screen role -- a performance that accomplishes more in limited screen time than many lead performances achieve in two hours. (wikipedia)

The Collins arc tracks across all five acts of the beat sheet

The beat sheet reveals that Collins appears in every act of the film:

• Act One, beat 3: Confesses she wanted to shake Carrie
• Act One, beat 8: Punishes the girls, bans Chris from prom
• Act Two, beat 12: Shows Carrie the mirror -- "That's a pretty girl"
• Act Two, beat 13: Interrogates Sue and Tommy about their motives
• Act Three, beat 22: Shares her own prom memory -- "It was magic"
• Act Four, beat 30: Dies calling Carrie's name

The arc runs from honesty about her own worst impulse to death at the hands of the girl she tried to save. Collins is the character who most clearly embodies the film's argument that kindness and cruelty converge on the same target -- she did everything right and died for it.