As it turns out, it wasn't any of his students at all! Well technically she was a student of his, but in school terms alone. The blonde-haired teacher of all things magical of Negi's, Evangeline McDowell stumbled on Negi's growth spell seconds before he was ready to cast it. The boy crept back a little when he finally noticed her coming up from behind him.
"What stupid thing are you doing now?" she asked him, an annoyed grimace on her face. "I hope you're not forgetting your training with me today."
"Of course I didn't, Master!", Negi quickly replied, hiding the circle in the dirt with his feet. Evangeline, however, just walked around him, wondering why he was playing in the dirt.
"What's with the circle, boy?"
"Um...."
"You're up to something. Spill it!"
And so Negi explained to his master everything he had learned recently about the growth spell he found out. The explanation proved pointless, though, as Evangeline herself had tried the spell decades ago, only to find it didn't work.
"It doesn't work", she told him point blank. "If I couldn't use it successfully, what makes you think you can? Even your father couldn't use it correctly!"
"Well I won't know until I try now, will I?" Negi asked her with a smile on his face. He then began the spell like he had planned to do so earlier, holding up his staff as he preformed the incantation of "Incresco Tantus!" All through it all Evangeline simply looked bored, yawning at the sight before her. She would've left right then and there if she didn't see a giant rock appear out of nowhere in front of Negi.
"What the...!?" the girl gasped at the sight of the 10 foot tall rock. "So it does work!" She then pointed at Negi, explaining to him, "You're going to try that spell on me next!"
"But Master! I don't know of a reversal spell...."
"Reversal spell!? Why would I not want to be bigger?!" she laughed at the child teacher, poking him on the head with her fingers. "Now just repeat what you did earlier, got it?"
"O-o-okay, Master...Just stand in the spot the rock was at earlier..."
Evangeline complied and waited for the boy to perform the spell once again. It worked perfectly, and the once short vampire could feel her body growing, laughing manically as her body pulsated bit by bit with growth. It didn't last very long, though, as the growth seemed to stop at 20 feet.
"Well that sucks", Evangeline sighed, hoping to grow bigger from the spell. "Still, this is definitely an improvement!" She then picked up the frightened Negi below her and stared at him with lowered eyebrows, smiling a disturbing smile at him.
"If you thought my training was tough before..." she teased the boy, slowly putting on the ground as she turned around in a full circle to show off her new size to Negi.
"Master, you can't possibly me to battle you at my current size! I have a hard time training with at your normal size!"
"That's true", the mini-giantess Evangeline admitted. "So....."