I have to continue my explanations here since I'm out of the description limits XD

SPOILER
The early period of Sheka (SheasXLuka pairing) is built on such a "hunt". Luka provokes and teases Sheas, but stops if he sees Sheas got scared. Every time Luka wants to make a further step, he waits for Sheas'es reaction (or at least lack of resistance). Luka's flirt is a problem for Sheas and it makes him to loose emotional stability and leaves him without his usual defensive shell.
At the same time Luka attracts Sheas and this turns into a war of heart and mind. Sheas understands he may not have relationship with his own teacher. But he wants to. This what makes him being torn between "want" and "may not". Luka has to use his intuition and guess where is real "no" and where Sheas tries to struggle himself. And since Sheas is very closed person, Luka has to be persistent sometimes.
END OF SPOILER

3) Why other students don't react Luka's behaviour?
Sheas is a student of the final year and the only newbie in class. This means all his classmates know Luka for a long time and aren't surprised with his behavior (it is most likely Luka has already flirted with everyone). This is why they may only find Sheas'es panic a funny thing. I would even say Luka's flirt is something like a rite of initiation