Is there any difference between "this is a friendly reminder" and "this is a kindly reminder"?
Which of them is proper to use when you use the phrase in email from teachers to student's parents?
Thank you very much.
"Kindly" can be an adjective, too. It means benevolent, gentle and agreeable. It's more often applied to people than to things, which is one reason why friendly reminder is the right choice in fujisan's post. The other reason is that "friendly reminder" is a stock phrase used for exactly this purpose - as a way of softening a reminder so that it does not sound like nagging. In fact, it is the usual phrase for this purpose, which is yet another reason it is the better choice.
Fujisan, if you reread post #4, you'll see that I suggest "friendly reminder." It is a common phrase used for exactly the purpose you intend. "Kindly" is usually applied only to people and, while not grammatically wrong, would look odd if used as you propose. Stick to friendly reminder - that's my recommendation.