Green Check marks in LoopMail is an indication that the message has been read. That means that the user logged into School Loop and viewed the message.
School Loop Plus
LoopMail also sends out email notifications when a message is sent. It is possible that others also have read the message, but did so in their email clients without logging into School Loop.
In that case, they would not show a green check mark, even if they did read the message. The checkmark is only for users who viewed the message through our system.