Why walk when you can teleport? {prologue}

"Excuse me," Caitlyn Brown muttered as she pushed her way through the crowd of boisterous middle school students. It wasn't like they were paying any attention to her, anyway.
Someone tapped her shoulder, and Caitlyn jumped.
"Oh, it's you." she said, relieved. "Thank goodness you're here! I could use the help. You busy tonight?"
Naomi Birch, the shoulder-tapper, bit her lip and nodded. "Yeah. What for?" she asked.
Caitlyn dropped her backpack to the ground and started shuffling through it. "I have this stupid social studies project that's due tomorrow. I haven't even started it yet!" she sighed and pulled out her folder, which contained the project assignment sheet, and passed it to Naomi.
"Tough luck," her friend agreed. "Yeah, I can definitely help. When d'you need me?"
Caitlyn zipped her bag closed again and swung it on to her back. "No clue. Maybe seven?"
Naomi nodded again. "I'll be there," she promised, smiling at Caitlyn, who smiled back.
"Oh no! I just remembered my brother has a soccer game tonight. I gotta be home by four!" she stressed. " I wish I could just teleport, y'know? "
Naomi looked at her strangely, and then her friend's mouth fell open. "Caitlyn..."she said, voice getting more and more distant with each syllable. " You're.." a look of confusion crossed her face.
 "You're disappearing!"