It was signed off by the Queen's lady-in-waiting with an official Buckingham Palace masthead.
Room 20 teacher Krystal Anderson said as a child she had written letters to the Queen with her mother and had always enjoyed receiving the reply.
"I always enjoyed it as a child and thought it would be a neat thing to do with the kids."
Ms Anderson started teaching at Otumoetai Primary at the beginning of the year but had written letters with seven of her previous classes at Auckland primary schools.
She said the children loved the experience and it was a good opportunity for them to learn about another country, the Royal family, as well as letter writing and sending.
With Queen's Birthday weekend this week, a boy came to class "quite upset" they had sent the card too early, she said.
"I had to give a big explanation about how she'd already had her birthday and was just having her party this weekend," she said.
The official website of The British Monarchy said the monarch could be contacted at the following address: Her Majesty The Queen, Buckingham Palace, London, SW1A 1AA.
"If you wish to write a formal letter, you can open with 'Madam' and close the letter with the form 'I have the honour to be, Madam, Your Majesty's humble and obedient servant'."