In HTML5 there is no difference, other than XML conformance which is not required when the document is served as "text/html".
As it stands now, text/html is the de facto MIME type, and therefore the default in which case, HTML 4 becomes the core specification, and valid HTML5. There are some exceptions to this, such as deprecated or obsolete tags and attributes.
<br>
is valid, as is,
<br/>
or,
<br />
The last is the specified form in XHTML, the recommendations of which HTML5 is a refinement. To be backward compatible HTML5 needed to support both HTML 4 and XHTML. It was XHTML that demanded the / in self-closing tags, not HTML 4. In fact pre-XHTML the term had not even been thought of yet. <BR> is a void element in HTML 4.
Bottom line, unless the lesson checker expects the /, it can be left out of all tags, this especially since as stated earlier, the page is served as text/html unless otherwise specified. Further reading… XML Conforming Markup Languages.