Biff entered the house before Heather did, it wasn't anything against her, but she certainly wasn't his type, his level, and he wanted to make the best impression he could. In truth it was Heather who had dropped back behind Biff for the same reason, she wanted to be seen as respectable and intelligent and hanging around a neanderthal like Biff was a long step away from that. She was doing this as a favor to her mother, not as something she wanted to do. For Biff this was a right of passage, one of the most important nights of this year, but for Heather this was an annoying event which was going to take up a night that could be spent on things that were more productive than popularity contests.
Biff was dressed up, which for him meant a button down collared shirt and slacks, which his football days had taught him was the way you dressed for important occasions. He looked good, every bit the all American guy, ready to nail an important night. Heather was dressed normal, which meant a floral shirt and capris with sandals. It's not that she didn't look good, she looked fine, but that was because she didn't care about any of this. This is why Biff didn't want to enter with her, in his eyes her priorities were all out of wack, she didn't seem to care about being popular or flirting with him, or having a good time and he didn't know how to react to that, much less be associated with it.
So they entered and went opposit directions, Biff to the guys, and Heather to the girls, as the room had been divided. There was in fact kept a strict line between the two by the senior members of the houses. Backs were to be turned at all times to the other gender, even thought everyone shared the same room, this wasn't about them. It was a test of commitment. What made things worse is the center of the room was divided by the drinks table. A further test. The message of the night was simple, joining up wasn't about getting drunk or laid (though it was implied you would), it was about the principals of the houses, which were boldly posted on the opposing walls which you were to face.
Biff hated this, and he squirmed as he focused on not looking at the sounds of girlish giggles and teases of shirts being removed, and even a bra being thrown over from the other side, to uproarious laughter from the girls. He forced himself to focus on what was in front of him, a pledge master barking about honor, fraternity, commitment. It wasn't what he signed up for, but he was in it for the long haul, he wasn't going to let his dad down.
Heather on the other hand found herself liking it. It was challenging, and strange. She didn't find it hard to avoid looking over at the guys, or not drinking, even when a she thought she heard the voice of a guy she liked calling her name out. She liked the idea of this thing being build on something other than shallow pursuits, and she found she believed in the ideas she saw on the wall, "Trust, Courtesy, Beyound Self in all things." It was the last one which stood out to her, because she didn't know exactly what that meant, and she puzzled on that idea as she watched other girls escorted out for failing this test. One that by accident she ended up passing.
And that is how both Biff and Heather made it into their Greek houses.