We Open to see (Y/n) at his original Age Standing at a stage and Singing as he slowly becomes older.

Hiroi aoi kono basho de
Tokiga nagarete yuku
My precious time

After the song said "My precious Time",We are given a Title Card showing the Fic's name of "Big".

Itsu no manika kiete shimatte kakegaeno koega
Tsumaranai hibimo sorega sukide ikitetanda

We transition to (Y/n) and his older Classmates Looking bored In class as well as Relaxing in their Dorm with Our Toddler Teen Smiling At the 4th Wall before Becoming Confused.

At tetanda,His Face suddenly split showing One half being Normal and the other Being Covered By darkness.

Higa sashiteru tokomade omoikkiri hashitte
Shinjirarenai koto bakari menomaeni okiru

It transition to show (Y/n) Hanging out with All of his Classmates and Doing all Child Like things with his teachers Groaning at them.

[Pre-Chorus]

Taiyou no hou wo miagete tachiagaruno tsurakete
Susume koto dake omouto

A Small Figure is Seen but no one Could Tell who it is as Everyone tried to Reach out for them and As it Rapidly changed from person to Person,Once it Reached Our Toddler Teen....

[Chorus]

Setsunai tabiwo tsuzukeru kawatta me no okuniwa

Issei was in His Balance Breaker Battling Vali as Bikou tried to Attack him from Behind but Koneko did him a Favor and stopped the Attack as Her Eyes Darkened with A Smirk.

She Grabbed him and Sent her Own attack.

Natsukashii itoshisa wo motomete (On my way)

It Changed to Izuku and Akko Having A good time Before Showing Battling Someone with Izuku Bleeding out.

Kizutsuke attanowa nikui wake janai nosa

It Rapidly Showed Allies And Enemies Before it Changes To (Y/n) On The Ground in His Original Body With Somebody Reaching Their Hand out to him as he looks up and sees Yagyu.

Bokuno subete wo Giseini torimodosunda Bokura no my precious time

He smiles and Grabs her hand as He Becomes our Toddler teen,We all know and love as The song ends with The Entire School Standing and Smiling at The Fourth wall.
__________________________________________________________________________________

Asuka and (Y/n) Walked through The Dorm Rooms Until they Stopped at a Room Door that looked like it was Built,Recently.

Asuka:Welp!here's Your Room,(Y/n)!

He went in and saw his New Room and Noticed It was Completely Empty.

(Y/n):Huh.

Asuka Says that it may be empty but he can Fill it up with Whatever and Decorate his room,however he wants.

(Y/n):Really?!

Asuka:Yeah!

He was excited as he never Decorated his room before but he's going to,Now......after he gets stuff.

Asuka Yawns as She Says It's Time for Bed and Heads to her room but Looks at Our Toddler Teen and said...

Asuka:You should head to bed,too....It'll be your first Day of Class,Tomorrow.

(Y/n) nods at went to his bed and went to Sleep while thinking....

(Y/n):'Tomorrow will be fun!'

He falls asleep and enjoys his Nap as he Enters the Dream World.

*Cue Timeskip*

The next morning,(Y/n) is forced to Wake up at 6 in The MORNING!!

(Y/n):*tired Whine*

Unfortunately for him,he was told last night that Students must get Up Early for His Classes.

So,he gets out of bed.....Reluctantly but He still does it and Goes down stairs to see his Classmates Ready for Class.

They all Turn to see (Y/n) in Nothing But Boxers,Which Immediately Caused them to Flip.

Jiro:AAAAAAAAHHHHHHHH!!!!

Issei:DUDE,PUT SOME PANTS ON!

Some of the girls That didn't get Freaked out From their New Classmate in nothing but Boxers,Just Gawked and Drooled at His Chest.

Mina:Momma Likey!

Sucy:You Can Grind Meat On Those!

Koneko didn't say a Word but she was Seen drooling and the same could apply to Hibari.

(Y/n) looked down to see Why they were staring and saw He Looked Buff.

(Y/n):Wow....I'm Buff!

They ones that wanted him to be dressed Yelled at him and Told him to put clothes on and After looking like he was gonna Cry,he did.

Now Dressed,The Entire Class begins Breakfast as Our Toddler teen Sees Some new faces.

??:Hey,Gray!what The new guy ust did reminds me of What you do.

"Gray":Shut up,Natsu!

At that,a Blond girl decides to point something out...

??:Gray......You're in your Boxers,Again.

The now named Gray looked down and shouted in shock as He actually was In just his Boxers,AGAIN.

After eating Breakfast and learning the names of the new people,they All Proceeded to Class.

Young (Y/n) was Basically Awful at Everything since he was Actually 4 Years old and wasn't supposed to Learn the High School stuff til he was An Actual Freshman.

But Combat Class is Where he shined,due to.........Actually,I'll just show you.

We see (Y/n) and his Classmates Sitting in Class Until It was Time for Our MC to fight and his Opponent.....

Shoto Todoroki.

The two arrive in the Arena and as Soon as Goodwitch said to began,Todoroki launched a Large Blast of Ice Towards Our Toddler Teen.

But As (Y/n) Stood there,He blew on his Thumb causing his Hand to Enlarge into a Giant Fist that he used to smack The ice away.

(Y/n):My turn!

He Walked towards Todoroki as the Dual haired Teen shot more Ice and some fire but it didn't do anything until....

Todoroki was sent flying into a Wall as Our Mc Smiled and said....

(Y/n):THAT WAS FUN!

Everyone else(Mainly Perverts):'Note to self,Do Not Piss him off'

Certain girls in Class:'But He can Smack me,Any Day'