How would I find all "Exams" that belong to a "Student"?

Leave class out of it, as the id is not relative to the class.

So:

user.rb model:
has_many :exams
belongs_to :class

exam.rb model:
belongs_to :user

controller:
user = User.find(current_user.id)
user.exams #=> [1,Math], [2,English], etc.

Cheers,
Zach Inglis
→ Blog -- http://www.zachinglis.com
→ Company -- http://www.lt3media.com
→ Portfolio -- http://portfolio.zachinglis.com