Why did Laurie decide not to join her friends in the stands at the football game?

Laurie goes to the football game against Clarksville. She looks forward to meeting Amy there and talking about The Wave and her relationship problems with David. However, when she tries to enter the stands, Brad tells her that she must give the Wave salute before she can be allowed in. She argues with him, and Brad seems to agree that the salute rule is silly. However, he is also afraid to let Laurie into the stands without enforcing the rule. He finally agrees to let her in, but Laurie storms off, scared and upset by the hold The Wave has taken on her classmates.