Hi, Denise. Unfortunately, there is not a way that you can mark a message as being unread. I have passed your question on to our developers, so you may see this featured added in a future update.
This would defeat the integrity of TalkingParents. The 'complete record' is just that - the read receipts are necessary so that one party cannot deny having seen a message. To have Kristin K. [TalkingParents Rep] suggest this as a possible functionality suggests that she doesn't know how TP works and doesn't inspire confidence in the TP platform.