Since it's declarative, it ends with a period. We should add some commas as well.
<em>S</em>chool began on a cold<em>,</em> <em>S</em>eptember morning<em>.
</em>
That should work.
John was disappointed by Sarah’s rejection of his proposal.
The commas must be placed both before and after the nonessential element. For example...
My mom, who hates cooking, ordered takeout.
"who hates cooking" is the nonessential element, and you can see that a comma goes before and after it.
It is ineffective support because it presents opininon rather than fact
