All the more certainly.
If drug users are going to be subject to mandatory sentences, then a fortiori drug dealers should be subject to them also.
A fortiori in Latin literally means "from the stronger (argument)." It is used when drawing a conclusion that is even more obvious or convincing than the one just drawn. Thus, if teaching English grammar to native speakers is difficult, then, a fortiori, teaching English grammar to nonnative speakers is even more challenging.