This isn't an error.
Symfony threats all classes as entities and if you're "mapping" them with doctrine you'll create the corrisponding tables onto database.
Now inheritance have to be taken into account: every "field" (property) into parent classe, will be extended or inherited by child.
So is perfectly clear that the corresponding parent field will be created into database.
To me the best way for solve this is to create a parent class and to migrate all commons (fields,methods and so on...) into it.
Then, you'll extend that new parent calss into user
with specific fields (in that case username
) aswell into student
with student's specific fields.