Perhaps headteacher might allow boys, except first-years, to put messages in (7)
I believe the answer is:
bottles
'perhaps headteacher might allow boys except first-years' is the definition.
I can't tell whether this definition defines the answer.
'put messages in' is the wordplay.
'put' becomes 'bottle' (bottling is a kind of putting).
'messages in' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'bottle'+'s'='BOTTLES'
'to' is the link.
Can you help me to learn more?
(Another definition for bottles that I've seen is " Containers for liquid".)