Tumbled over boarding train with possible classmate (12)

'possible classmate' is the definition.
(someone at school with you)

'tumbled over boarding train with' is the wordplay.
'tumbled over' becomes 'fello' (I can't justify this - if you can you should believe this answer much more).
'boarding' is an insertion indicator.
'train' becomes 'school' (both can mean to teach or coach).
'with' becomes 'w' (abbreviation).
'school'+'w'='schoolw'
'fello' put into 'schoolw' is 'SCHOOLFELLOW'.