Alice and Rudy were so lost in kissing that they didn't pay attention to their surroundings. They forgot that they were in a classroom with its door open for anyone to walk in. And even if no one walked in, they could still be seen from the doorway if any student or teacher passed through the hallway.
In all that, a group of their female classmates was coming towards the classroom as one of the girls forgot her money in her bag.
Sure, Rudy had super senses, and he could hear if someone was approaching them, but he always shut off his senses and powers whenever he was doing things with his girls, as he didn't want to accidentally hurt them.
But that wasn't the only reason why Rudy didn't notice a group of his classmates was coming their way. He could have still heard the sound of footsteps if Alice wasn't covering his ears with her hands.
She was holding his head while covering his ears and used them to pull Rudy close to her as Rudy played with her breasts.