OP, I had exactly the same situation last year at my DD's CE school. To the school's credit, they listened to my point of view, and stopped supporting OCC/Samaritan's Purse. Here's what I wrote - feel free to use any bits you like and good luck!
"I understand that the school is intending to support Operation Christmas Child. I'm glad that the children are being encouraged to think of others less fortunate than themselves. However, I have profound reservations about the charity, and the appropriateness of the school's supporting this particular appeal.
I am not sure if you are aware of the controversy surrounding Samaritan’s Purse. The founder of OCC, Franklin Graham is blatantly intolerant and insulting of other religions, demonstrated by such quotes describing Islam as “a very wicked and evil religion”. On Hinduism he says “…we travelled to the subcontinent of India, with its hundreds of millions of people locked in the darkness of Hinduism… These people were bound by Satan’s power”. I also cannot ignore his homophobic beliefs, demonstrated by his support of Vladimir Putin's draconian anti-gay laws and his racist belief that "black men get shot because they don't obey law enforcement officers".
While the vast majority of those involved in Operation Christmas Child are likely acting with good intentions, the scheme is highly problematic - not only on account of the organisation's aforementioned bigotry - but because of the way it seeks to use poverty and humanitarian tragedies to evangelise to children. The charity Save The Children has questioned its effectiveness and expressed concerns about the use of evangelism in the context of people in need. Also, filling shoeboxes with plastic trinkets to send overseas is an horrendously inefficient way to give to charity. Reputable charities usually seek to convert physical donations into money, because money can be used far more effectively. Real charity should be unconditional with no strings attached, yet OCC state that those who do not accept their message are damned to eternal punishment.
I feel strongly that this charity has extreme values which are diametrically opposed to my own Christian values. I do think it is a fantastic idea for the children to be involved in giving and thinking of others, particularly at Christmas time, but there are many alternatives to Samaritan’s Purse."
Here are some links to articles that might be of interest:
www.theguardian.com/commentisfree/belief/2014/dec/12/evangelical-giftwrapping-islamophobia-marches-on
www.newstatesman.com/religion/2013/10/operation-christmas-childs-shoebox-campaign-just-propaganda-tool-christianity^