Hurry up selected pupils to start working regularly (4,2,6)
I believe the answer is:
come on stream
'start working regularly' is the definition.
(come on stream means to start working)
'hurry up selected pupils' is the wordplay.
'hurry up' becomes 'come on' (They mean the same).
'selected pupils' becomes 'stream' (group of pupils with similar ability).
'comeon'+'stream'='COME ON STREAM'
'to' is the link.
(Other definitions for come on stream that I've seen before include "(Of oil wells) start regular production" , "do as well initially?" , "become productive" , "Start to produce" , "Encouraging words to pupils in group" .)