It was a typical day at U.A. High School. The students of class 1-A were enthusiastically looking forward to their usual, exciting day of learning and training. This day had more in store for them than they could have imagined.

Their homeroom teacher, the stern but fair Aizawa, had assigned them an important essay the previous week. The students were expected to turn in their completed work at the start of class.

As Aizawa was making his routine check, he frowned when he reached Denki Kaminari, one of the more outgoing, but occasionally clueless members of the class. "Kaminari, where is your assignment?" He asked sternly.

All eyes turned to the electrifying quirk-user who rubbed the back of his neck awkwardly. "I...uh...ate it," he finally admitted, causing the room to fill with stunned silence.

Aizawa blinked, once, twice. "You did what?" he asked in exasperation, likely expecting a joke or a metaphorical explanation. But Kaminari simply replied, "I ate it, sir."

Laughter spread through the classroom, each student finding the situation more ridiculous than the last. Yet Kaminari maintained a completely serious expression, which only added to the hilarity.

Even Aizawa's facade cracked. The usually stern-faced teacher failed to stifle an amused smile. He tried to deliver a stern rebuke, but it was hard to find the right words.

"Kaminari, you can't just... eat your homework. That's not how this works," he finally managed to say, amidst the sound of his students' laughter.

"But you said it was a piece of cake!" Kaminari protested, looking genuinely confused, which only set his classmates off into another round of guffaws.

In the chaos of amused disbelief, Mina Ashido managed to gasp out, "Are you telling us you've never heard that phrase before?" Denki looked baffled. "What phrase?" He asked innocently.

Aizawa sighed, running a hand over his face. "It's an idiom, Kaminari. When someone says 'it's a piece of cake', they mean it's easy, not that it's edible." He explained, trying not to laugh.

This new information seemed to blow Denki's mind. His eyes widened and he slapped his forehead in realization. "Oh! That makes so much sense!" He exclaimed.

The classroom erupted into another round of laughter at Kaminari's revelation, even Aizawa shook his head in amusement before regaining his stern expression. "Next time, Kaminari, ask when you're unsure about something."

It took a while for the class to regain their composure. In a classroom full of teenagers endowed with unique powers, situations like these were oddly refreshing. They created a sense of normalcy amidst the extraordinary.

This incident ended up being a highlight of their day. It became a running joke amongst the members of Class 1-A. Anytime a student faced a seemingly difficult task, they'd just reassure each other saying, "Don't sweat it, it's a piece of cake."

Kaminari, despite his initial embarrassment, took it in his stride. He would feign confusion and ask, "Can I eat it?", which always induced laughter, lightening up the mood.

It was friendly ribbing and harmless fun, a temporary diversion from the high stakes of hero training they were undergoing. Moments like these reminded them of the lighter side of their high school lives.

Besides, it served as an important lesson for Kaminari. From that day on, he was more meticulous about understanding idiomatic expressions, wanting to avoid any such comedic mix-ups in the future.

It wasn't just Kaminari who learned a thing or two that day. The other students had a refreshing reminder that they might be training to be heroes, but they were still teenagers.

Aizawa too, despite his initial frustration, enjoyed the comic relief. It reminded him that beneath the superpowers and hero aspirations, his students were just ordinary teenagers, learning their way through life.

In the grand scheme of things, this was just another day at U.A. High School. But for the students of class 1-A, and especially Kaminari, this was one day that would be remembered with a chuckle.

From then on, the phrase "it's a piece of cake" had a double meaning in Class 1-A. It wasn't just about something being easy anymore; it was a lighthearted memory of one of their classmates' innocent mistake.

And as for Kaminari, he sure will never look at a piece of cake the same way again. Or, for that matter, any homework assignment. He learned his lesson, and it was equally delicious and embarrassing.

This was not just an inside joke for the students; it was a moment that strengthened their bond as a class. Amidst the constant battles and strict training, moments like these brightened up their daily routines.

And so, the tale of Kaminari's delicious homework became a part of Class 1-A's lore, a story to be retold with a fond grin, and a reminder to never take idioms too literally!