I Can't Help But Feel Left Behind

We were sitting at dinner listening to the brunette twin talk about her school day. She was talking about Fast Track math when the blond twin's eyes teared up and she said, "I can't help but feel left behind because I'm not in Fast Track math." Then she started crying.
I held her while she asked why she wasn't in the class. She wanted to know why she wasn't smart enough. Daddy and I said all the right things about class limits and not enough space. She eventually stopped crying, but barely.
This is the problem with having the girls in different math classes. I completely understand that the blond twin didn't quite score high enough to make the cut. She barely missed it, but she missed it. We have to manage her disappointment without letting her think that she's not good enough to be in the class. We don't want this to put her on a "I'm not good at math" path. That would be the worst outcome of all.