He cuts it off her

(*parts borrowed from spiralfelix and aligned for quality continuity.*)

*Mackenzie's heart pounded and her mind raced. Fuck the test, fuck whatever the hell's going on, I just need to get the hell out of here! was all her mind could come up with. That was when the redhead realized that she was still leaning forward on her forearms. And, more importantly, that was when she realized that her skin-tone bra was still unclasped. It remained vertically dangling straight down from her shoulders, leaving Mackenzie's slanted torso free and clear of its last form of cover. Her voluptuous breasts were in view for the class to see, clear as day. Well for anyone seated to the left or right of her at least. But those who luckily were in those prime seats they were treated to a wonderful side view of Mackenzie's boobs.

The redhead involuntarily stiffened in shock as she realized her newfound exposure, quickly leaning back so she was sitting straight up again. Her bra cups obeyed gravity and swung back onto her breasts, providing what meager cover they could for the nearly naked girl. As a blushing Mackenzie self-consciously crossed her hands in front of her chest, she covertly looked around the room, and she didn't like what she saw. Mackenzie noticed that almost everyone was now looking right at her! Her face reddened even further as she resolved to, at the very least, refasten her bra.*

At least, that was going to be the plan. Zack wanted to wrap this up quickly. He didn't have to do well on the mid-term to pass, but he did have to DO part of it. So, for alacrity, he pulled out a pair of scissors he had for his art class later. As the bra lay loosely on her chest, he reached forward with the scissors, sliding them carefully so the strap was ready their maw.

Snip
Mackenzie gasped as she felt all tension go out from her right strap and saw/felt it fall away down over her left forearm. What!?
Snip
Her eyes went wide as she felt all tension go out from the left as well and saw/felt it fall over her right forearm this time. NO WAY!? She screamed internally. She clutched the bra to her chest with her arms, trying to keep it in place, aware that uncrossing her arms would likely cause her to flash her boobs to the class again.

Zack took this opportunity to wait for a moment where the professor wasn't looking, and he grabbed the forgotten clasp hanging from the left and pulled. Hard. After a moment of resistance, the garment slid left and was whisked away from Mackenzie's grip and with a quiet "eep!" the alluring redhead was suddenly naked in class, with her arms barely covering her bare breasts. Zack slipped the bra into his bag, before settling down to take the test with one last glance up and down Mackenzie's body.

*And that was it. Mackenzie was left wearing nothing but her socks and shoes, despite having walked into the classroom an hour earlier wearing a complete outfit! She couldn't believe it. She was stuck taking her midterm naked!

This is a nightmare, she thought to herself. This has gotta be a nightmare! There's no way this is really happening, right?! But after a few more moments of failing to wake herself up, Mackenzie slowly began to accept her reality. She debated what she should do in an unimaginable scenario like this, and only one answer came to her mind. She had to get out of there!

Mackenzie grabbed the test and her small backpack, her only two forms of cover now, and held them tight against her pale skin. Her backpack was thankfully large enough to cover both her breasts when she hugged it to her chest, but not much else. Her test, likewise, was strategically placed in front of her crotch, hiding her womanhood from view for the moment. As Mackenzie turned to her side and stood up from the desk (trying not to think of how half the classroom was now getting a spectacular view of her curvy ass) she caught the eye of the boy behind her. Zack smiled and winked at her, happy to have helped the "shy streaker" in her stripping escapades. Mackenzie blushed and glanced around his desk, desperately hoping to find one of the pieces of clothing he'd stolen from her. But Zack had stashed his prizes too well, and they were all hidden out of sight in his bag.

With no small amount of resignation, Mackenzie turned to the front of the room and began hastily yet quietly walking up to the professor's desk with her meager forms of cover. And even worse, upon arriving, she had to relinquish one of them. The naked redhead plopped her exam onto the desk, causing the professor to look up from his book just slightly for a moment before he resumed his reading. Not wanting to spend another second in that room, Mackenzie covered her pussy with her hand, and headed for the exit as quickly as she dared. She expected to suddenly hear the professor shout something at her, but he mercifully seemed too distracted to even notice the naked girl just feet away from him. The same wasn't true for the rest of the class, however, and every student in the room was transfixed by the sight of Mackenzie's pale skin and uncovered ass as she fled the room.*