Do you have a rapid assessment tool in the district to use? That question may be one to ask the head of counseling, just so counselors and nurses are using the same tool.
If no: you want one - and googling rapid assessment suicide or mental health will land you a few good ones.
It is my personal practice to assess for lethality and ability. "I feel like killing myself" is not by itself lethal. "I'm going to jump off the bridge onto the highway," "I have pills at home," "My dad has a rifle" are all examples where I'd never leave the kid alone, not for a minute. Not even if I had to summon the superintendent.
Brief physical assessment - any cutting marks, evidence of past attempts? I'm only looking at what I can readily see. No taking off pants to look at thighs (someone else can do that).
I call a parent every time and discuss assessments, observations, and referrals. If I believe the student is actively going to harm him/herself I will get district assistance to transport that student to the parent's place of choice- it's ridiculous for me to send a parent and kid to the ER if the kid is going to jump out of the car at a stoplight or some such.
Do you have a resource officer to help with an involuntary placement? What's your local intake/inpatient place like?
I look forward to hearing more about what some of our former MH nurses would add here.