I have found that if you make TinyMCE your preferred Text Editor.
Usermenu > Preferences
You can make some reasonably better changes in the HTML part of the Label settings page.
If you add the following into the first <p> opening tag like this...
<p style="margin-bottom: 0 !important;" >
it will remove the unwanted margin from each label that you add that style to.
I think you will find that works for an editing teacher.