Kaat Benahid, Marrakech 40000, Morocco
9am - 7pm, to 6pm winter (the opening hours listed on the official website are not accurate)
Go early in the morning, and aim to get there for opening, before the scorching sun comes up and the Madrasa fills up with other visitors.
Ben Youssef Madersa was an Islamic college, and is now a museum fully open to the public. I've visited before on my last trip to Morocco, and couldn't wait to visit again. You can read the post from my last visit here.