School is a public place.
Further the principal demanded she provide proof.
Further, almost all two-party recording states make exceptions for people to record conversations if they believe illegal activity is going on.
And that is exactly what she as doing. gathering evidence of illegal treatment of her son for a principal that demanded evidence it ws occurring.
Further the teacher denied doing it when she did do it, and the evidence confirms it, and that the teacher is a liar and has destroyed her credibility to be trusted.