gara = rattle (of Iwamoto-sensei arriving, presumably)

ga = impact (Iwamoto hitting Yuusuke. This is the point at which your humble editor tried to strangle the manga. According to the ever-helpful M.J. Johnson, hitting students, at least in the old-fashioned sense of a whack on the knuckles or head, is not frowned upon in Japan and indeed supposed to help the learning process.)

Shuku and San-kan = an extra panel by Togashi-san