Nearing his ninth decade, formal in vested suit and cufflinks, Rabbi Haskel Lookstein strode purposefully into a classroom of Ramaz High School on Manhattan’s Upper East Side one recent Monday afternoon. He checked the presence and location of his 18 students against a seating chart. He chided one for arriving moments late.

Then he led off the discussion of the homework assignment. It consisted of an article from the national Jewish newspaper, The Forward, about a married couple who participate avidly in both synagogue and swinging. “Aren’t these people just being honest?” Rabbi Lookstein asked. Five or six hands immediately shot up.

So began another day in Jewish Sexual Ethics, the course better known around Ramaz, even to its teacher, as “Sex With the Rabbi.” For the last 23 years, since Rabbi Lookstein devised the class, he has taught it to every 10th grader to pass through Ramaz, a Modern Orthodox institution combining rigorous secular and religious curriculums.

“This is one of the most favorite things I do in the world,” Rabbi Lookstein, 77, said in a recent interview. “I love the interaction with students  and being able to open their eyes to the way in which Judaism approaches the basic facts of life.”