At the top of their class but all on their own and left out (4)

'at the top of their class but all on' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'own and left out' is the wordplay.
'own' becomes 'one' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'left' becomes 'al' (I've seen this in other clues).
'out' means to remove the last letter.
'al' with its last letter taken off is 'a'.
'one' after 'a' is 'AONE'.

'their' is the link.

Can you help me to learn more?