The first step in sending a broadcast to a specific list of students is to gather a CSV File of just 1 column of student ID's of students from your building. Please take a look at this
example file for more detail.
(Make sure when saving, you choose CSV - comma seperated from the file type).
Next, once you have this file. Login to SchoolMessenger and click "New Broadcast". Once you are there, this quick video will show you how to upload your list of students to school messenger.