Make a mistake getting schoolchildren upset (4,2)
I believe the answer is:
slip up
'make a mistake' is the definition.
(slipping up is a kind of making a mistake)
'schoolchildren upset' is the wordplay.
'schoolchildren' becomes 'pupils' (I've seen this before).
'upset' says the letters should be written in reverse.
'pupils' backwards is 'SLIP-UP'.
'getting' is the link.
(Other definitions for slip up that I've seen before include "Make a mistake, being unsteady on feet?" , "one fielder close to the wicket?" , "Pupils (anag.)" , "get it wrong" , "Small error" .)