Although the audience doesn't find out Taiga has to transfer schools until episode 24/Book 10, she found out about it when she visited her mother after the ski trip. Episodes 22-23/Book 9 show her acting based on that knowledge. Her refusal to complete the career assessment? It won't matter because she'll be off to another school next year, anyways. Her conviction that Ryuuji can still get Minori? Once Taiga moves, there'll be no reason for Minori not to go out with him. The chocolates she gives everyone? They were meant as farewell presents. Had Minori not interrupted, Taiga probably would've told everyone what was going on.
The only thing written on Taiga's letter-of-apology to Sumire was "Baka". Considering that she called her and her friends idiots, but later indirectly confessed of her affections to Yuusaku by lamenting that she also wants but cannot allow herself to become "an idiot who only knows how to dash forward" like them (as she knew Yuusaku would do anything to follow her all the way to America had he known), it was the highest compliment Taiga could give her.
The opening dialogue in the first episode, beyond its repetition in the finale, gets an easy-to-miss reference in the first of the Beach Episodes. The opening dialogue deals with Ryuuji and Taiga's thoughts on 'something that can't be seen', while Ryuuji and Minori's discussion in the Beach Episode touches on the issue of ghosts, which Minori describes as something she'd never seen. The latter came about as an answer to the question of whether Minori had ever loved anyone while it's pretty damned obvious what that 'something which couldn't been seen' is in the opening dialogue.
Ami has blue hair and Minorin has red hair but everybody freaks out when Yusaku dyes his hair blond, and then they try to forcibly dye it black again. (Although, Blond dye is considered a hallmark of being a delinquent in Japanese culture, so there is at least some reasoning behind it.)
Ami (and the other blue-haired characters) can get away with it if you consider blue to represent a different shade of black. Minori, on the other hand, is a bit harder to justify. And then there's Haruta whose hair apparently is blue in-story, since he's the one Kitamura goes to to learn about hair-dying.
Minori can be just like Ami, a lighter coloured brown that's been stylised much like blue. Blue is a shade of black, Red is a shade of brown.