How did she keep a straight face

My guess is that she's female and the joke is more for men. Or maybe she just kept her eyes focused on Principal Carter. Remember, they weren't really laughing that hard until that end of the scene and that's when she got up and left. Still, to this day, I think that's one of the top 5 funniest scenes of all time.

I thought I saw an out-take somewhere where they keep trying to keep straight faces and she loses it along with everyone else in the scene. Multi takes.

I rewatched the scene today. It was on HD TV so I had a nice and clear 1080p picture. You can see that she's trying VERY hard not to laugh. You can clearly see her crack a smile 3 times. The 3rd time she smiles she almost looses it. So, she closes her eyes and forces her self to frown. That just managed to save her because that was right before she got to walk off camera. I'm sure the second she walked off camera she erupted into a laughing fit.

I had that experience once. When I was still doing sub work in the local school district I was sent to this continuation jr high. One kid kept bugging me to let him blow up a condom. I told him no and next thing I knew when my back was turned there was a balloon condom bouncing around the classroom. I wrote out a referral which I sent with him to the Asst Principal. It was all I could do to keep a straight face till he was outside. Then i lost it. Afterwards leaving the school I was still laughing. When the staff asked what was up I told them. He had told them he embarrassed the daylights out of me.Not a chance.