What is a personal data breach?
A personal data breach means a breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to, personal data. This means that a breach is more than just losing personal data.
Source: https://ico.org.uk/for-organisations/law-enforcement/guide-to-le-processing/personal-data-breaches/#:~:text=A%20personal%20data%20breach%20means,than%20just%20losing%20personal%20data.
"a small number of parent or teacher users could be served up with the incorrect information" sounds like a data breach considering the nature of the incorrect information.