ED fully self-contained classroom often becomes the default placement for children with IEP who pose danger to others. However, parents have the right to disagree with district's proposed change of placement and until the dispute is resolved, the stay-put provision is in effect. The only way she could land in the ED setting over her parents' objection is through due process except for a few extreme exceptions including serious bodily harm to others. This is why the recent downward spiral in the K readiness class was worrisome. Threatening another student with a pair of scissors is just one of those things that raises a red flag.
I personally believe that IDEA gives parents a lot of power and tools to seek out FAPE for their child. It's been a while since I've sat down and read the relevant case law and regulations so by all means, don't take my words for it
For more details:
IDEA discipline IDEA discipline Q & A more on discipline 
