・・・「、。！」
Ding-dong… dong-dong… "Whoa, not good. That's the main bell!"
Literal
Ding-dong... dong-dong... / Whoa, bad. Main-bell is!
キーンコーン・カーンコーン is the stock Japanese representation of a Western-style school bell (the four-note Westminster chime). 本鈴 ('main bell') is the actual start-of-class signal, as opposed to the 予鈴 ('warning bell') that rings a few minutes earlier. まずい ('bad') is a versatile adjective that flips between 'tastes bad' (food) and 'this is bad / we're in trouble' (situation); context disambiguates.