Cyberattack is a fairly generic term (as is anything "cyber-"). Just intentionally joining a Zoom room you're not supposed to be in/know about is probably "unauthorized access to a resource" and thus a "cyberattack" according to some definitions, and mainstream media, school administrators, ... are going to use those kind of definitions.

E.g. random definition pulled from merriam webster dictionary:

> an attempt to gain illegal access to a computer or computer system for the purpose of causing damage or harm

One can argue that there's not enough focus on the attack on a system here, but I'm not surprised it's used that way.