I sincerely cannot imagine what was going through their minds. In addition to the ableism there’s an incredible lack of self awareness on display here.
Given that they offered the same choice to the parents of the disabled kids, it sounds like a genuine fuck up rather than anything sinister.
Probably just some office admin not realising that some kids were missing and thinking it was just two arrangements of the same class. It's not like it's some fine art bullshit. They're doing dozens of these a day.