Signal pupils to assemble perhaps: sound familiar? (4,1,4)
I believe the answer is:
ring a bell
I believe this clue is a double definition.
'signal pupils to assemble perhaps' is the first definition.
(as in a school bell)
'sound familiar?' is the second definition.
(something which seems familiar rings a bell)
(Other definitions for ring a bell that I've seen before include "Signal to start class" , "Seem familiar; toll" , "Formally declare (appointment)" , "Advice to cyclist" , "jog memories" .)