Do I keep it really minimal or... Well, let's add some punctuation and link words and see how it comes out.
Pupil droplets: in ones and twos down the streets and lanes they gather into threes and fours, gradually form streamlets which coalesce into brooks and then into rushing burns as the school gates are reached, pour in and swirl and whirl and gradually pause, find their level in the reservoir of class or hall. But this river is nothing compared to the bursting of the banks and the torrent which outpours when the final bell sounds.
The underlined words are where I feel unsure / unhappy. Two graduallys in one short poem is one too many! Should I use the word 'dam' at some point? Would that be better than banks - which sounds good but is less accurate.