If it were for school, I would answer just as the others did encouraging you to display something about New Years, and would just add that today Japan displays more Christmas illumination than it used to 17 years ago, thanks to the Japanese invention of blue LED. But since it's for church, I'm going to give you a different answer.
Christmas in Christian churches in Japan is quite different to what's going on at hotels and restaurants in Japan. It is very religious and humble, and more focused on mutual understanding of the society.
I am not baptised but I like to go to the midnight Christmas service whenever I can. At all the churches I've been to, which are usually Anglican or Catholic, Japanese Christians come dressed in jeans and sweaters: What matters is not what they wear, but how they pray. When I was younger, I would run to church after partying with my friends, and I'd be the only one there who's drunk:)
At a Catholic church I went to a couple of times, at the middle of the Christmas service, those who've gathered are encouraged to bow to the people standing on both sides of your seat, which in my case were always total strangers. As you may know, bowing is the most typical way to greet Japanese people, so I guess it's like shaking hands or hugging someone if it were in a church in say America.
This Catholic church is quite a big one, and after the service, you were free to have some hot soup outside. I recall the soup was ton-jiru or something as Japanese as that. Very typical for any winter event in Japan, except for the commercial Christmas festivals. This is the church's website;http://www.svdtokyo.jp/index_e.html
On Christmas Eve, NHK TV news always broadcasts a short footage of a church in Nagasaki, because Nagasaki has the longest history of Christianity in Japan. Here again, people would be dressed in simple clothes but would wear lace on their heads as they are conservative Catholics. This is Nagasaki City's official website;http://www.at-nagasaki.jp/foreign/english/spot/
Hope it helps.