A possible version：
Boys and girls, may I have your attention please? I have an announcement to make. About fifty American students will come to visit our school on the morning of November 15th. We’ll welcome them at the school gate at 8:30 and then have s series of activities. The arrangements are as follows: Firstly, we’ll show them around the campus. And then, a meeting will be held at the conference room, where we can communicate with them and discuss about how to learn English well. Last, we’ll take photos together to memorize this special occasion. Those who want to take part please sign up at the Student Union before November 14th.
That’s all. Thanks for your attention.