I'm not a lawyer, and I disagree.
Board members should be allowed to express opinions and discuss board business by email, including sending email to all the other board members and using "Reply All". This would save considerable time, because they can be educated between meetings. They should be careful not to solicit voting commitments or to "survey" other members about future votes.
Such emails would be subject to disclosure under FOIA requests, and such emails should not be used to avoid discussion in public at board meetings.
All board members now have and should be using the District's official email system. Personal email accounts should not be used to discuss official school district business.
Because two board members used personal email addresses for 4-10 years, the school district should be proactive in securing those GMail accounts and preserving ALL emails related to official business. NOW is the time for the District to take action on this. It should not wait for any future FOIA request. Those two board members should not delete any official emails from those accounts -ever. And they should not close those email accounts.
That problem with those two personal email accounts could have been fixed four years ago, but it wasn't.