Legally they probably can. It's been police procedure, and it may be worth checking with the local district as to their policy about talking to children. I suspect they would want to notify you. But things that your child says may be used against them, even if they are interrogated without a parent, at least in Florida. A good defense attorney may be able to keep statements out of court, but it would be an uphill battle.
The provisions in criminal court protecting minors from questioning without a parent being present relate to cases in which a child confesses to a crime, not to an investigation. That is - the confession could be suppressed from evidence.
In your situation, there is nothing to suppress because your son was not the subject of the investigation. You should talk to the school about their having allowed it, and possibly contact a civil attorney if you feel the police behavior was in some way bad enough to warrant damages.