Is This Ever Going to End? Pecking Order Question

This is my first year raising chickens. I have three of them. One, Penny, is almost six months and the other two are almost three months. This age difference is because we had to get rid of the older chicken's pal, since he turned out to be a rooster. I did try to get chickens that were closer to Penny's age, but they weren't available. So we brought the two little chickens home, finished raising them in the brooder, and then approached putting them out in the pen with Penny.

I knew about the pecking order and all that, but I didn't have any idea how brutal it could be. Penny chased the younger chicks around, cornered them so they were cowering in a corner, and then kept threatening/pecking them. Naturally, I stopped that behavior. I tried to approach it as a territorial issue, so for a week, I brought the baby chicks out and let them be near each other outside the pen so they could get used to each other. Penny got a little better, but she still chased them and pecked at them. The young chickens became *terrified* of her and shriek when she came near--who can blame them?

When it came time to put the baby chicks in with Penny, we made a see-through screen and leaned it against the pen wall so that a section was walled off. That way the younger chickens could be behind the screen in the pen and they could see Penny (and visa versa), but Penny could not hurt them because she doesn't fit under the screen.

That was two weeks ago. The chickens seemed to be getting a little better--they don't hang out together in a bunch, but they do peck near each other when they are outside the pen, and once Penny defended the young chickens from the cat. On Saturday, I took the screen out and left them in the pen together. The younger chicks are full pullets now, and they are so afraid of Penny that she doesn't even have to peck them, so between the two things I figured that at least it was physically safe. So I left them together and then on Monday, I realized the Penny was spending most of the day backing them into the corner so that they practically flatten themselves down to get away from her. She doesn't hurt them, but I don't think it is good for any creature to be constantly terrified, so I put the screen back in.

I am sorry this is long! Anyway, I want to ask: do you think this will get better? Is this going to be their permanent relationship or do you think it will be worked out in time?

Also: should I take the screen out and let them work it out, or should I leave it in until the younger chickens are as big as Penny?

It makes me sad to see the younger chickens so terrified. But Penny is already laying an egg a day, so I must be doing something right!