I can tell you, schools get so many unsolicited emails a day...

I can tell you, schools get so many unsolicited emails a day (that breech Australian Spam laws) that aren’t read and are deleted. Doing a mass email out is easy and cheap to do but isn’t legal unless the recipient has agreed to receive such emails. The Australian Spam laws exist to prevent exactly what you just suggested.

Again, working via Principal Associations can overcome this.
Through my membership of the South Australian Primary Principals Association, I’ve consented to them emailing me and know that they will promote sponsors occasionally. They’ve been upfront about that.