Police questioning students at school.

I'd advise your child to basically just ask 3 things. 1) Am I a suspect of a crime? 2) Am I being detained? 3) What is the nature of your questioning and probable cause? If he's too young for those kind of questions, I'd advise the same as you have, except adding that if they won't let him call you or that teacher, then refuse to comment.

you should begin with the school policy first. The school board determines when and where the outside officials come in - otherwise, they settle things "in-house.'

If police have been called in to a school setting, you are talking about serious stuff i.e. drugs, weapons, etc.

Yet another student has been suspended for having something that represents a gun, but isn’t actually anything like a real gun.
This time, it was a breakfast pastry.
Josh Welch, a second-grader at Park Elementary School in Baltimore, Maryland, was suspended for two days because his teacher thought he shaped the strawberry, pre-baked toaster pastry into something resembling a gun. WBFF, the FOX affiliate in Baltimore, broke the story.
Welch, an arty kid who has reportedly been diagnosed with attention deficit hyperactivity disorder, said his goal was to turn it into a mountain, but that didn’t really materialize, reports Fox News.
“It was already a rectangle. I just kept on biting it and biting it and tore off the top of it and kind of looked like a gun,” he said.
“But it wasn’t,” the seven-year-old astutely added.
The boy’s teacher was not happy with his creation.
“She was pretty mad, and I think I was in big trouble,” Welch told the FOX affiliate.
According to the boy’s father, school officials say Welch also said “Bang, bang” while holding the breakfast pastry.
School officials sent home a letter saying, in part: “One of our students used food to make inappropriate gestures.”
Beyond the letter, school officials offered no further comment on the incident, citing privacy concerns.
“They said they had to suspend Josh for two days, because he used his breakfast pastry and fashioned it as a gun,” the elder Welch told WBFF.
The boy’s father described the events leading up to his son’s suspension as “insanity.”
No one was hurt during the incident, he noted.
“It’s a pastry, you know,” he said.
Josh Welch is adamant that he didn’t say “bang, bang.” He does admit pointing his breakfast pastry sculpture at the ceiling.
This incident is the latest in a growing line of extraordinarily strong reactions by school officials to things students have brought to school — or talked about bringing to school, or eaten at school — that are not anything like real guns.
At Poston Butte High School in Arizona, a high school freshman was suspended for setting a picture of a gun as the desktop background on his school-issued computer. (RELATED: Freshman suspended for picture of gun)
At D. Newlin Fell School in Philadelphia, school officials reportedly yelled at a student and then searched her in front of her class after she was found with a paper gun her grandfather had made for her. (RELATED: Paper gun causes panic)
In rural Pennsylvania, a kindergarten girl was suspended for making a “terroristic threat” after she told another girl that she planned to shoot her with a pink Hello Kitty toy gun that bombards targets with soapy bubbles.
At Roscoe R. Nix Elementary School in Maryland, a six-year-old boy was suspended for making the universal kid sign for a gun, pointing at another student and saying “pow.” That boy’s suspension was later lifted and his name cleared. (RELATED: Pow! You’re suspended, kid)
In Sumter, South Carolina, a six-year-old girl was expelled for bringing a clear plastic Airsoft gun that shoots plastic pellet to class for show-and-tell. The expulsion was later revoked.

In my reading I've learned that kids are often questioned about things while at school -- even things that didn't happen at school -- because it's easy to find them there and because kids are more likely to talk in a school setting.

I think questioning authority is important on a lot of levels. I think it helps kids AVOID getting into trouble in the first place if they are used to questioning people, including themselves.