Slime all over school beams (6)
I believe the answer is:
smiles
'beams' is the definition.
(I've seen this in another clue)
'slime all over school' is the wordplay.
'all over' is an anagram indicator.
'school' becomes 'S'.
'slime' with letters rearranged gives 'smile'.
'smile'+'s'='SMILES'
(Other definitions for smiles that I've seen before include "expresses pleasure" , "Shows signs of pleasure or amusement" , "Looks happy" , "Facial expressions showing pleasure" , "Beams, grins" .)