In spite of her little "strageddy" (to borrow a word from a certain wascally wabbit), Hikari-chan wasn't fooling Sensei at all. Turns out that Sensei was on to her from the get-go - plus she (figuratively and literally) "poked the sleeping dragon" until it awoke.
Luckily for her, Sensei had feelings for her (personally, I commend him for aiming to hold back until she graduated ... but a healthy young man who's reached his limit in that way has little choice but to engage "beast mode").
Ah, sweet Vanilla goodness.
So they're both actually just really shy and couldn't come out and tell each other how they feel straight up, eh? I like it. Well, he did mention he planned to confess after she graduated, but clearly she couldn't wait that long.
And hey, it ended up working out really well. A nice, simple release and I like it.