Spec style syntax with minitest in rails

According to the spec section within the minitest docs: the gem allows you to either use the "unit_test" syntax of testing or the "spec style" syntax of testing.

In my rails app: I have two tests on a scope within my model (note: I am using factory_girl to build objects within my tests). The following is, as I understand it, the typical way of writing tests with minitest within the context of rails:

# test/models/city_test.rb
require 'test_helper'

class CityTest < ActiveSupport::TestCase
  test ".with_active_schools should return none" do
    city1 = create(:sequenced_city)
    city2 = create(:sequenced_city)
    create(:sequenced_inactive_school, city: city1)
    create(:sequenced_inactive_school, city: city1)
    create(:sequenced_inactive_school, city: city2)
    create(:sequenced_inactive_school, city: city2)
    assert_equal(0, City.with_active_schools.size)
  end


  test ".with_active_schools should return all" do
    city1 = create(:sequenced_city)
    city2 = create(:sequenced_city)
    create(:sequenced_active_school, city: city1)
    create(:sequenced_active_school, city: city1)
    create(:sequenced_active_school, city: city2)
    create(:sequenced_active_school, city: city2)
    assert_equal(2, City.with_active_schools.size)
  end
end

I then run rails test test/models/city_test.rb and all tests pass.

Now I want to see if it is possible to convert the above tests to the spec-style syntax. I comment out everything above in the same file. I then replace it with this code:

# test/models/city_test.rb
require 'test_helper'

describe City do
  describe ".with_active_schools" do
    it "returns no cities with active schools" do
      city1 = create(:sequenced_city)
      city2 = create(:sequenced_city)
      create(:sequenced_inactive_school, city: city1)
      create(:sequenced_inactive_school, city: city1)
      create(:sequenced_inactive_school, city: city2)
      create(:sequenced_inactive_school, city: city2)
      expect(City.with_active_schools.size).to eq(0)
    end
    it "returns cities with active schools" do
      city1 = create(:sequenced_city)
      city2 = create(:sequenced_city)
      create(:sequenced_active_school, city: city1)
      create(:sequenced_active_school, city: city1)
      create(:sequenced_active_school, city: city2)
      create(:sequenced_active_school, city: city2)
      expect(City.with_active_schools.size).to eq(2)
    end
  end
end

I then attempt to run the tests: rails test test/models/city_test.rb, however it errors out:

undefined method `describe' for main:Object (NoMethodError)

I am not sure why it is throwing this error when the minitest docs says it supports this syntax. I did look at the rails guides for testing, but there is little content within the model testing section.