Look! The bus . . . . . . . . .

The given sentence is written in the present continuous form of the tense.
It is continuous, as the name suggests, and represents an event that continues to happen in the immediate future or the present.
Such sentences can be structured by adding an auxiliary verb like is, am or are along with adding 'ing' to the verb.
Here, the auxiliary verb is 'is' and the verb becomes 'leaving'.