Why don't you call the School or Principal to understand more. May be the information that your elder kid is studing in S1 has been missed out somehow. It's nothing to lose for just a call.
Last year I went there meeting Principal at around 0800 to 0810.....
Better hand in to him directly( my suggestion)....Actually, the key point is not on the content of letter. It is to let Mr Lo to have your image printed in his mind....