I'm working on a product that is aimed towards schools, there was a email list online for plenty of schools. Am I legally able to send a mass email to them promoting and telling them about our product?
Are there any specific rules I have to follow?
The CAN-SPAM Act of 2003 contains some rules that you're required to follow: provide an unsubscribe option, use a non-deceptive subject line, include your physical address, no fake headers, etc.
As far as I know, it is legal to send any email to any email address you have.
But the downside of spamming is not on the legal aspect. Spamming will get your brand banned, blacklisted and listed as spammers by email servers. Most email servers updates their banning list to a global directory of spammers and scammers list. Much like virus updates by anti-virus software. It is to keep their users out of the danger zone. Email spamming is not a good choice in marketing. Please consult email marketing services for more advice upon this mater.
Opt-in is the best technique. -- Create a website that tell people what you do. Attract them to join your mailing list with value propositions (VP) and unique sales proposition (USP) as well as attractive benefits. See: UVP
Then start your marketing program that drive your leads towards your goal. Either using banner ads, fb ads, offline marketing, publishing, media ads etc. Then funnels them towards your objective. See: Marketing funneling.