Why am I not seeing the option to Post - I am a teacher/ admin but still not seeing the option to post when I click on the +Create. 

 This happens when the post/ announcement feature for your class is not turned ON. Please follow these steps to turn it on. 

Click on the Class

Click Settings icon

Click Edit Settings 
Click Features 

Turn ON the Post/ Announcement feature 

Click Save 

Now you should be able to see the Post option the Create menu.