I have no idea if this still works, and it probably wouldn't work for a school, but 15-20 years ago the way to get Yahoo to stop blocking your emails was to call up your ad rep and say something like "Why the heck should we keep spending $10000/month buying ads on Yahoo when any new customers we get from those ads that use Yahoo email end up pissed off at us and maybe even charge back because it looks like we are completely ignoring them because they don't see our emails????".

That would get you added to a "never block mail from this domain" whitelist that had higher precedence than everything else.