I could be completely wrong, but the quote about 'mind-numbing' assemblies makes me think of Secret of Susan. I think Mary Anne said something like that to Kristy before they went to the assembly where Kristy randomly noticed the handicapped kids for the first time. I don't have a copy, though, so I can't check.
You're right! This sounded like it (I had forgotten about this one since it's one of my least favorites) so I checked and Kristy says it before the school assembly. Thanks!