Assuming that the listeners here are the students, the following can be said.
First, whose 笑顔 is being talked about? The students', agreed? If yes, the phrase "your smiles" might make an appearance in the next sentence. This being Japanese, however, its "appearance" might as well be invisible.
The next sentence 「油断させるには足りませんね。」 is a complete one though it lacks both a subject and object. You might want to fill in the missing parts either on paper or in your head. (Nothing is missing to the Japanese-speaking mind, needless to say.)
「(Subject) + は + (Object) + を + 油断させるには足りませんね。」
「油断させる」 is causative, so the Object is the one who may or may not 「油断する」, right?
Thus, it is saying that the Subject is 足りない ("not good enough") to make the Object 油断する ("to be off guard").
Subject: 「(Your) わざとらしい笑顔 or 笑顔のわざとらしさ」
A good translation has already been given in the comment section above. If the listeners are indeed the students, change the "their" in that TL to "your".