Embedded below is a 51 second YouTube video on how to do this followed by step-by-step directions beneath the video.
When you have the post the way you would like it - simply select the class next to where it says For (under announcement)
Then, select the classes that you would like to share the same information with.
You can also send specific posts to individuals in an individual class by selecting All Students and then selecting the specific students you want to send the post to.
Any questions, let me know!