She follows the rest of the class out, only remembering her things on sheer autopilot. Several classmates ask what that was about.

"I wanted people to pay attention to me."

They stare at her. She  immediately launches into an unrelated running commentary about her weekend plans. She's talking louder than necessary and basically only making it to the lunch room because that's where the crowd is headed.. 

She has a packed lunch so she can squeeze into a crowded table and insert herself into the conversation. The consumption of lunch is going to be slow. and she keeps trying to talk with her mouth full.

They're not all looking at her and it burns. There's barely room for anything else in her head. She shouldn't scream, but maybe if she screams loud enough they'll pay attention and it will stop hurting and she'll be able to think.

A kid finishes lunch and puts on a pair of noise cancelling headphones and closes his eyes.

her last nerve  S N A P S. All she can think about is the kid brazenly ignoring her. If they're going to ignore her she'll make them all pay attention.

She screams in the kid's face and before he can react grabs a bottle of hot sauce and squeezes it as hard as she can manage directly in his face.

(fortunately for both parties in this interaction the hot sauce was only mild hot sauce and there was no permanent damage done)

The floodgates thus opened she starts grabbing every object on the table within reach and hurling it into the crowd.

After a minute the football coach manages to get to her and start carrying her off to the office, but not before the table is thoroughly trashed. 

She screams at the top of her lungs all the way down the hall. This hurts of course but the pain is nothing beside the pain of being ignored. (Everyone in the hallway is looking at her but they could be paying more attention to her, right?)

The vice-principal scolds her. The principal scolds her. The secretary gets her mom on the phone and her mom scolds her.

"You don't understand, they were ignoring me. I had to make them stop."

(some tiny part of her brain knows she's not really making any sense, but it's drowned out)

Eventually they sit her down in a side room to wait to be picked up and lock the door.

She screams at them to come back in. She bangs on the door until her knuckles bleed but it's sold enough would that she cannot (yet) break it.
After a little while she gives up. Obviously they've all forgotten she exists. Probably everyone has forgotten she exists. Probably nobody would notice or care if she was dead.

After a little while longer Meghan's mom comes in to find her sitting on the floor, with bloody knuckles, staring into space. Anger instantly melts into concern and she kneels down to hug her daughter.

"Meghan, sweetie, what's wrong? What's going on?"

It takes her a little while to react. Her mom is here? Her mom knows she exists?

"You have to tell them I exist. You have to tell them to pay attention to me."