Here's a hint: to do your job. If you believed that placement in a special education classroom was necessary, then your job was to demonstrate that that placement is necessary, not to railroad the parents. If the placement was necessary, it could surely survive some scrutiny from "a lawyer" during an assessment or IEP meeting. Rumor has it that school districts and Departments of Education even employ their own laywers versed in IDEA.
How would you go about demonstrating that the placement was necessary? And do you really believe that just because you've provided evidence of such necessity, the parent's won't threaten to sue regardless, wasting massive amounts of money? From what I can gather in Loughla's post, he/she teaches in a small community, and so their school department may not have the funds to mount a proper legal defense, especially seeing how the autistic student's parents were willing to spend the money to hire a lawyer from quite far away. The whole point of these legal threats may just be to say "i can waste a lot of your money", as to dissuade any real action, even if the reason was legitimate... because let's face it, frivolous law suites are not at all rare in this country.
Egotist: A person of low taste, more interested in himself than in me. -- Ambrose Bierce