I came upon these two sets of lists, 13 Things Your Child's Teacher Won't Tell You and 20 More Things Your Child's Teacher Won't Tell You.
I agreed with, and enjoyed, the second list more.
Here's one I liked from the first list:
Your child may be the center of your universe, but I have to share mine with 25 others. (Darren's note: if it were only 25 others!)
Here's one from the second:
The truth is simple: Your kid will lie to get out of trouble.
Here's a bonus one from the second:
Parents give their kids the pricey gadgets and labels, but what kids really crave is for you to talk to them. They want to know you are interested in their lives.
It's true. We teachers may discuss these amongst ourselves, but we usually won't tell parents these things.