Rails Pass A Parameter To Conditional Validation - ruby-on-rails-3

Question

I'm importing heaps of student data from an spreadsheet document. Each row of student data will represent a new user, however, the possibility of importing an already existing student exists and I want to bypass some of my user validations such as username uniqueness accordingly so that I can build associations for both new and existing records, but only if they're being imported to the same school.
Thus far I have the following validation setup in my User model:
user.rb
validates_uniqueness_of :username, :unless => :not_unique_to_school?
def not_unique_to_school?
user = find_by_username(self.username)
user.present? && user.school_id == 6
end
Now how would I go about replacing that 6 with a value I have access to in the controller? Instructors will be the ones handling the importing and they'll be importing students to their school so I would typically run current_user.school_id to retrieve the school id that I want them to be imported to, but I don't have access to the current_user helper in my model.
I'm not concerned about duplicating usernames as I'll be handling that on a different step, this is just the preliminary validation.
Edit
Simplified school & user model:
user.rb
class User < ActiveRecord::Base
# Setup accessible (or protected) attributes for your model
attr_accessible :email, :password, :password_confirmation, :remember_me, :username,
:first_name, :last_name, :school_id, :roles_mask
belongs_to :school
validates_presence_of :username, :on => :create, :message => "can't be blank"
validates_uniqueness_of :username, :unless => :unique_to_school?
def unique_to_school?
user = find_by_username(self.username)
user.present? && user.school_id == 6
end
def find_by_username(username)
User.where(:username => username).first
end
end
school.rb
class School < ActiveRecord::Base
attr_accessible :country_id, :name, :state_id
has_many :users
end

Answer 1

I'd add a method to your School model:
def student_named?(name)
self.users.where(:username => name).any?
end
then in your validation:
def not_unique_to_school?
self.school.student_named?(self.username)
end

Answer 2

Here's what ended up working for me:
validate :user_cant_be_duplicate_in_other_schools
def user_cant_be_duplicate_in_other_schools
errors.add(:username, :taken) if User.count(:conditions => ["school_id != ? AND username = ?", self.school_id, self.username]) > 0
end
As opposed to testing if a User belongs to a particular school we're testing for the lack of belonging to a particular school. I didn't come up with this answer, another user posted this as an answer but deleted it shortly after for reasons unknown.