Sans couldn't believe his luck, he'd hoped for this but didn't expect it to actually work out.
The portal had appeared right in the middle of UA's oval, Aizawa, followed by a large group of other heroes rushed out to meet them. Sans dropped Flowey off and moved to the front of the crowd next to the king.

"don't worry, these guys are with me." Sans called out to the group, Aizawa looked relieved same as the rest of the heroes. "asgore, these are ua's staff. they'll be helping get everyone settled. everyone else, this is asgore, the king of monsters." Nezu popped out from Aizawa's scarf, before dropping to the ground. The rodent man stepped forward and offered his paw.

"It is an honour to meet you sir, I'm Nezu, principal of UA!" He introduces, the two begin to chat, Nezu explained his plans for housing the three hundred plus monsters and teaching them about how to live in this world.

"SANS! OH MY GOD, WE'RE ON THE SURFACE!" Papyrus shouts, sweeping his brother up into his arms.

"yep, told ya it'd work." Sans replied, his brother placed him down again. Sans could help but stare, Papyrus was free, everyone was free, and for good this time. Hopefully.

"WHAT'S THAT GIANT BALL?" Papyrus asked, pointing up to the afternoon sky.

"we call that "the sun," bro." Sans explained, Papyrus' eyes light up with excitement.

"THAT'S THE SUN!? WOWIE!!! I CAN'T BELIEVE I'M MEETING THE SUN!!! HELLO SUN!" Papyrus waved at the glowing ball of light, staring at it. Good thing he doesn't have actual eyes to burn out.

"Everyone!" Asgore shouted across the swarm of monsters. "This is the beginning of a bright new future. An era of peace between humans and monsters, with their help, our species will be integrated into their society. However, we have been locked away for many years, the humans have adapted and changed. Human magic has adapted and changed, instead of being able to cast spells like in the days of old, their magic has conjoined and changed to create one source. Some even look like us, which will make it easier to integrate." Asgore spoke loud and clear. "Until laws are understood and adapted, we will be staying here, at UA high school, as the humans work to make this world a shared home between us." Asgore gestures to the school standing tall in the distance. "Come now, my fellow monsters, let us go to our new home." He turns to the group of heroes. "Lead the way." Everyone cheered loudly and excitedly as Nedzu led the way to their new life.

Sans wanders to the back, it seems Aizawa had the same idea as they both meet once the swarm is gone.

"this is gonna be a hectic next few months." Sans sighed with a smile, glancing up at the taller man.

"Indeed." Aizawa answered flatly. "So, what's your plan?"

"what do ya' mean?"

"I mean, going forward, what are you going to do? Keep going to UA? Focus on your people? Or family?"

"i'm not gonna put my time here to waste, it'd be illogical to drop outta ua. as for the dorms, i plan to stay with my bro for a while. he'll probably be stuck there since we're both underaged." Sans hummed thoughtfully, Aizawa combed a hand through his hair.

"Well, if you're under my legal care, shouldn't he be as well?" Aizawa asked, Sans let out a short chuckle.

"so, what you're saying is you're going to adopt a third- or is it- fourth kid?" Sans smiled.

"I have been needing to move into a bigger place for a while now." Aizawa hummed, Sans paused for a moment.

"oh, that reminds me." His hands sunk into his pockets, scrummaging around. "we talked about this briefly during a meeting, but-" Sans handed Aizawa some gold. "-that should help with the costs." Aizawa just stared at the gold bars he held in his hands, he didn't let it show, but they were cumbersome.

____________________________

It's been a few months,  a lot has happened, some monsters have been able to move out of the dorms to live everyday lives, others, it was best to stay on campus.
UA did their best to prepare them for this world, and they did a really good job, most monsters fit in seamlessly.
At the moment, Undyne was trying to get into a hero university, Toriel was training to become a teacher, and Asgore opened a flower shop while also acting as the ambassador for the monsters. Alphys was still working as a scientist, a lot of humans knew about and wanted to work with her. Mettaton still had his show, it was one of the most popular in all of Japan.
All the kids were going to school now and living a normal life, and the rest of the monsters were just living their lives.

Papyrus got to move in with Aizawa, he was over the moon to meet the man and Eri. Sans made sure to introduce him to his classmates and teachers. One of the most notable interactions was with Present Mic. He offered Papyrus a place on his talk show, he accepted on the spot. 
So, Pap was an internet sensation.

Frisk was still locked up, and as long as Sans lived, that's how it was going to stay.

And Sans, well, he was doing what he does. Once Papyrus moved to Aizawa's, Sans moved back to his class dorms, none of them really knew what was going on, and Sans did his best to avoid explaining anything but a basic lie all staff had agreed upon.
Flowey was one of the very few remaining people living at the school, so naturally, Sans'd make sure to bother the flower a lot.
And by bother, he meant hang out with him. Yes, Sans disliked the weed and sometimes felt like squishing him. But Sans made a deal, and it was a deal he planned to keep. The two actually talked quite a bit since one had nothing better to do. 
To Sans' surprise, creating the soul didn't take as long as he'd thought, a few months. Flowey was more so gone, Asriel was back, but there were some complications. Asriel wasn't sure what to do.
At the moment, he was still avoiding his parents, avoiding the humans outside of the school and avoiding confronting things. No one could make him do anything, he had to do it himself.

Still, I'm sure he'll do it eventually.

When he's ready.

You know that saying of 'all good stories must come to an end.'

Well, congratulations.

You've reached it.

The ending.

There is nothing more to read.

Nothing more to the story.

Is this a happy ending?

For some.

And even if there are still things that need to be done.

This is the end.

That's what I would've said if this were the end.

It's true what I said earlier, that all good stories must come to an end, and that applies to this story too.

But.

Not.

Just.

Yet.

...

=)