@pinkeree: No, it's the 'he's not acting differently because he's ALREADY in love' option.
@SummerK:
She's the only girl he take initiative to approach (everyone else approach him first)
At the graduation party, the teacher's magic sent him to her (yes, it's that kind of magic, that's why he's like "why to where you (Nunnaly) are")
That whole sequence of the air dance (also note how he went quiet the moment she smiled)
She got in through the barier that 'only let those trusted in'
He recognized her disguise on the spot + hid her in a rather intimate manner when the disguise came of
Even his dad chose her because he could read how Alves is treating her differently.
It basically started out as 'boy tease girl he likes for attention' that evolve into 'guy resigned his waifu is too dense for romance tease her because he feels it's unfair he's the only one melancholic in this' with the touch of that stubborn 'refuse to confess first because it'd feels like he lost and she definitely will act haughty about it'