To share a photo or create a new note about your child, you need to select the "Share a post" button , located in the lower section.
In the next screen, select your child(ren) in the "Tag" field, and then you can write your post and/or add a photo.
If you you to share the post with your child teacher, select the "Yes" option in the "Share with teachers" button.
By default, the option is not selected so your post won't be shared with the teacher, just with your friends and family.
Then you just need to select the "Submit" button and everyone in your group will receive your post.