On Frisk's first day of school, they shared many classes with some monsters. One of the benefits of kindergarten is children judged on actions, not your species' deeds.
But teachers weren't children. "Frisk." The teacher took them aside during recess. "Is everything at home okay?" Frisk nodded. "Where'd you get that cut?" The teacher asked, pointing to Frisk's arm. "I fell on the sidewalk. Hurt. Grillby helped me fix it up." The teacher hummed.
Not that wasn't the only racism Frisk noticed. They noticed any schoolwork a monster did was marked stricter. They thought that was wrong.
Humans usually were formed in a group containing just them, and monsters with monsters.
Frisk asked Grillby, "What do I do if someone is treating monsters wrong?" "What do you mean?" And this is how Grillby ended up sitting in on a meeting with the principal.
The principal feeling bad for Frisk did go through the motions of making sure everything was marked fairly. Though, yep, it was stricter, it was nothing the monsters couldn't handle.
But to be inclusive, he told the teacher to mark things without looking at the kids' names. This seemed to make Frisk happier. And the teacher let them choose their groups. "Poor kid." The principal mused. Nodding, the teacher agreed, "Manipulated to help. Maybe they'll see it eventually."