Immature group of pupils in cockpit (10)
I believe the answer is:
greenhouse
'cockpit' is the definition.
(type of cockpit with multiple windows)
'immature group of pupils' is the wordplay.
'immature' becomes 'green' (synonyms).
'group of pupils' becomes 'house' (pupils often belong to school houses).
'green'+'house'='GREENHOUSE'
'in' is the link.
(Other definitions for greenhouse that I've seen before include "Horticulturalists' glass building" , "Glass building for rearing plants" , "associated with harmful gases" , "somewhere hot" , "see-through construction" .)