The action is as follows:
First fake an itchy crotch. Give a few credible scratches so that if you are later caught in the act you can say you were merely scratching yourself.
Then, after attracting the student's attention, tug twice on your penis. (N.B. It's important to keep the genitals inside the pants as removing them could attract attention.)
After tugging twice make sustained eye contact and give an inviting look. This should be accomplished by raising the eyebrows while simultaneously tilting the head back.
Stu Dent: What, is he hitting on you again?
Monroe: Yeah, I looked over at him during the test just long enough to see him give me the old Two Tugs and a Look
Stu Dent: I thought Mr. Stevens just had itchy balls.