Make a mistake about schoolchildren (4,2)
I believe the answer is:
slip up
'make a mistake' is the definition.
(slipping up is a kind of making a mistake)
'about schoolchildren' is the wordplay.
'about' says the letters should be written backwards.
'schoolchildren' becomes 'pupils' (I've seen this before).
'pupils' back-to-front is 'SLIP-UP'.
(Other definitions for slip up that I've seen before include "Minor inadvertent mistake" , "Minor lapse" , "Maybe literal" , "Make a mistake, being unsteady on feet?" , "Make a mistake or lose footing aloft" .)