You don't send a six-year-old to a mental institution because she throws a temper tantrum. Especially not if it's the school's, not the parents' choice.
Send her home, certainly. Refuse to allow her back until her "temper issues" have been dealt with, maybe.
For God's sake, people: she's six years old. She shouldn't be expected to act like a tiny adult. Her parents maybe should discipline her better, but temper tantrums and meltdowns are normal for that age, especially when the kid's scared, uncomfortable, frustrated, or tired.
Here's my question: what did the teacher ask her to do? Was it something that the little girl was having a frustrating level of difficulty with? Why didn't the teacher ask her what was wrong, and try to help before taking her to the office and handcuffing the child one day, and institutionalizing her the next?
The more I hear, the more determined I become to homeschool my child(ren).