To answer your first question, I have seen many videos of police interrogating minors without parents present. If a child is in custody, he or she has the same rights as an adult who has been arrested (the right to a lawyer, right to remain silent) and must have these rights told to them. Whether or not a child is in custody is something the courts and lawyers have argued about for years, but basically comes down to whether or not the a reasonable child in his shoes would feel able to leave. If he would, then he is not in custody.
If you are concerned that police are talking to your child while you are not there, you can advise him or her to be polite, but refuse to speak to the police officer without you or an attorney present. Police don't like that, but it is your child's right, and can protect your child when you're not there.
I am unsure what to say about the second part of your question. Are there specific instances of violence, threats, etc that are happening at the school that you feel are a threat to your child? You can certainly call the police and report the issue yourself, or have your child do the same.
Keep in mind that, depending on your child's year in school, he or she may be crossing into the realm of adulthood very shortly. In Wisconsin, children enter the adult criminal system at the age of 17.
If you have additional questions, please feel free to post a follow-up to this message. I hope I've addressed your concerns.