You can make a post directly in the Group's News & Updates page, inside the group. You can also make this post to your group from the School Feed.
Route 1: Go to your Group page
Go to Groups.
Select your group.
On your group page, click News & Updates. (You are automatically taken to this page if your group already has one or more post in its feed. Otherwise navigate from the Info page to this page.)
You do not need to select the audience. It is pre-selected for you.
Type in your post and click Submit.
Route 2: Go to School Feed
Go to School Feed.
In the Add post interface, click the text, "What would you like to share?"
Select your group name from the Audience drop-down.
Type in your post and click Submit.