Seesaw is set up on the assumption that it's easiest for everyone to rely on Seesaw instant messages instead of Email, but not all teachers agree.
If you DON'T want parents to use Seesaw to send you messages, and you'd rather they use Gmail (or Google Chat) you can disable the reply feature, BUT you have to disable it for every single family member, one by one, by switching OFF the option to 'Allow Replies in Inbox".
Once you've done that, they will no longer be able to send you messages in Seesaw...
https://help.seesaw.me/hc/en-us/articles/115003336743-How-do-I-disable-replies-