They secure pupil's head in form (6)
I believe the answer is:
clasps
'they secure' is the definition.
(clasps secure something in place)
'pupil's head in form' is the wordplay.
'head' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'form' becomes 'class' (synonyms).
The initial letter of 'pupils' is 'p'.
'p' going within 'class' is 'CLASPS'.
(Other definitions for clasps that I've seen before include "Embraces, hugs" , "Holds the fastenings to one's bosom" , "Holds or embraces closely" , "Holds tightly" , "Fasteners, as on necklaces perhaps" .)