... as they did in this particular case -- you could whisper to your aide to activate one of the plans you've already worked out in case of emergency. (20 seconds?)
You could applaud vigorously as the student who's reading aloud to you pauses for breath, and tell the class that there's some super-urgent business to see to (or if you want them to laugh, tell them you have to go to the little President's room). Smile, stand up, and calmly exit. (That should take 60 seconds, max.)
Once you're out of the kids' sight, hustle to the car and get back to AF1 at top speed -- if this is the start of a war, an elementary school isn't equipped with the weapons and communication equipment you'd need.
I wish that they'd showed ALL the classroom footage. In some situations, 7 minutes isn't a lot of time (say, if the boss is coming down the hallway and you've got to get that presentation ready) -- but paramedics and nurses, for example, have to make critical decisions on that time scale. They can and do, because they train for it and are prepared -- something which surely ought to be the case for national security as well?
This is how the widow of Flight 93's first officer felt:
"Mr. Bush was notified 14 minutes after the first attack, at 9
a.m., when he arrived at an elementary school in Sarasota, Fla. He
went into a private room and spoke by phone with his national
security advisor, Condoleezza Rice, and glanced at a TV in the
room. Mrs. Homer's soft voice curdles when she describes his
reaction: "I can't get over what Bush said when he was called
about the first plane hitting the tower: That's some bad pilot.
Why did people on the street assume right away it was a terrorist
hijacking, but our President didn't know? Why did it take so long
to ground all civilian aircraft? In the time between when my
husband's plane took off
and when the second plane
hit in New York <9:02 a.m.>, they could have turned back to
airfield.""
http://www.ratical.org/ratville/CAH/M.A.Sweeney.txt
I don't know if she's seen the movie yet, but I thought about what she said while I was watching that particular segment.
p.s. the only recent emergency I've experienced was finding a colleague who has diabetes, experiencing an insulin reaction. I got her to sit down, and found her some apple juice to drink. That took just under 5 minutes in total.