Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.  

----------------------------------

Ever since that whole incident with that other boy, Leo really hadn't enjoyed going to kindergarten. He had just had a little outburst, that was all, but he got the feeling, (even for only being a 6 year old), that everyone was scared of him now. Even when he was minding his own business, drawing, or playing with someone, there was never really a time when he couldn't feel someone's eyes on him, and when he would turn to look, anyone he might have caught staring at him would just quickly look away. The only person who wasn't really avoiding him like the plague was his sister. At least she wasn't afraid of him.

"Leo, can we join you to draw?" He heard his sister's voice ask him, as he was sitting at a small table in the room where he was doing his own drawing. Luna was standing next to another girl - her name was Sam. She had her blonde hair pulled up into a pigtail, and she had dark brown eyes. She seemed to also like to dress in bright pink overalls a lot with light pink shirts and shoes, (or perhaps her mother liked to dress her like that). Sam looked like just a normal human. There weren't actually any other pupils in the class that were both human and monster. Their class tended to include one or the other, and Luna and Leo were the only two who were even considered different to everyone else. 

"No, I don't want to play with him." Sam's voice spoke, sounding scared as she went to grab onto Luna's wrist as if she wanted to tug her away. "He's scary... His eyes just stare..." Sam murmured. That was the con with having a skeletal body and face, but for some reason he possessed blue, human looking eyes which he had obviously gotten from his mother. Even though he didn't have any skin, he also did have dark brown hair atop his skull. The whole thing with his eyes, it wasn't like they were too small for his sockets, because they were a fine size. It just probably wasn't every day you saw a skeleton with normal eyes. Sometimes he wished he had the same eyes as his sister, but he wasn't as lucky as her. 

"Don't be mean." Luna pouted at her friend, going to tug her wrist away from the other girl's grip. "He's my little brother. He's not scawy." Luna went on, wanting to stick up for Leo, as she didn't like that he was getting picked on by the others. Especially a girl who she had just wanted to play with. "If you're going to be a big meanie to my brother, then I don't want to be your fwiend." Luna finished, going to sit down at the table with her brother. Leo stayed quiet, but he simply watched the girl in the bright pink overalls stomp away, looking upset and stroppy about what her short lived friend had said. 

"Thank you." Leo softly spoke towards his sister, who smiled at his words, knowing that he was thanking her. After all, their parents, (especially their mom), had been doing their best to teach them about manners growing up. Seeing as his sister had done a nice thing for him, he wanted to show that he was grateful. Still, he hated that now he felt like the odd one out within the classroom. Everyone thought he was a freak now, everyone but his sister. 

"That's okay. What she said was mean." Luna said once more, beaming kindly towards Leo, but then her smile fell when she noticed how glum her brother appeared to be. "Don't be sad." She spoke up, causing Leo to look up from his piece of paper on the table with a saddened expression on his face. "Mommy and daddy always say that I'm your big sister, so I'll always look out for you!" She promised happily, hoping that would at least cheer him up a little bit. It was thoughtful on her part, but the little boy felt like he would be much happier once they were back at home, away from kindergarten, away from all the stares. He didn't like it here. 

When noticing she couldn't cheer Leo up, she too fell quiet, not sure of what to tell him. She hoped that maybe their mom and dad would be able to make him feel better. Even being so young, she liked looking out for him, because he was her little bro, (she was likely going to be even more of a protective sister when she was older). 

Finally, the end of the school day for them arose, and Leo felt at least somewhat relieved when their mom and dad had come together to pick him and his sister up. Normally, he wasn't really much of a clingy child, but the moment he saw them arrive he just wanted to run up to them and be held in their arms. Because he hated kindergarten, he absolutely hated it. And when asked by Frisk if the two of them had a good day, he only started to cry. Because no, he hadn't had a good day. Sans frowned. 

"hey, bud, what's wrong?" Sans asked his son who he was holding in his arms, and Leo just seemed to want to cling to him like his life depended on it. He wondered what made him so upset, but when glancing to Luna, she looked pretty sad too. They must have had a really bad day... The skeleton exhaled. "come on. let's get ya two home." When they were home, that was when they could perhaps have the conversation about whatever was the matter. Though... Sans had a feeling he knew what was wrong. Ever since Leo had flared up in anger in class, he had noticed how the little boy hadn't been enjoying kindergarten as much. Everyone probably looked on at him with fear just because of one small thing. 

"I get the feeling they're really not enjoying kindergarten lately..." Frisk sighed, once they were home, and the two parents were standing in the doorway of the living room, overlooking their children watching a movie while being wrapped up in blankets. Sans nodded, knowing he had been thinking the same thing. "...Maybe it's because they're different. I just didn't know children could be so cruel at such a young age." Frisk murmured with a miserable expression upon her face. Bullying felt like something that could only happen in primary or highschool. But apparently not. 

"...did ya ever have to deal with bullyin' when ya went to school, frisk?" Sans realised that in all of the time he knew her, they never really did discuss her school life and what it was like, before everything went to shit and she was forced to the streets. He never forced her to talk about that stuff because he didn't know if it was too sensitive for her, just learning about her parents being murdered before her when they were first dating had been shocking enough. 

"Yeah, I did. But who doesn't? I'm sure you used to get picked on when you were in school." Frisk said, only because it generally tended to happen, and she wasn't surprised when the skeleton did nod at that. There was always bound to be someone who would try to pick on you, because they thought you were weaker than them. It was stupid, really. "It really made me hate school - at least at first. Then I gained friends who would actually stick up for me and be there for me, and things became much easier after that... I hope they gain a lot of friends who are able to do that for them too." She said. Only because her and Sans wouldn't always be able to be there for them forever, and she never wanted them to feel like they were alone. That it was them and everyone else. 

"i hope so too." Sans nodded, knowing he also wanted the best for them. 

They both hated to see their children suffer in such ways.