You can make a post directly in the Group Feed, inside the group. You can also 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 Feed.
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.