When writing the letter there are a few important points you must take into consideration. Make sure you write in a very respectful and formal tone. So do not tell the teacher what is happening, write the letter in a way that suggests you are writing in order to seek their approval first. Also, make sure you include important facts such as the date your son needs off and for how long. If the vacation time is due to an important occasion such as a family wedding, it may be worthwhile mentioning this.
When writing the letter you could follow a template like this:
For the attention of Mr/Mrs/Ms ______ (insert teacher’s name)
Re: Request for vacation leave for ______(insert your son’s name)
I am writing to you in order to request some vacation leave for my son, ______. Due to a family wedding/Christening/etc, it is necessary we travel as a family to______(location). As a result, my son will need some time off school between (dates).
Although I understand how important his education is, I would not make this request unless it was necessary. I hope you can permit him to have this short time away from his studies. If it can be arranged, he would welcome having some extended homework to take with him during this absence from school.
I would like to thank you for considering this request and look forward to hearing from you soon.
______ (your name and title)