What language was your class using? There are so many ways to do what the students wanted to do, be it the tertiary function or a lambda function or even, defining a method/function like (Ruby):

  def foowiz(x, threshold = 5)

    if x == threshold

      foo

    else

      bar

    end

  end

And then a call to func(foowiz(x)) would work - even in an OOP language like Ruby.

What did the students need to unlearn?

In the intro CS class I took an age ago, we had to implement OOP as a final assignment in Dr. Scheme, which is a derivative of LISP that is most assuredly a functional language rather than OOP. That did not mean I had to unlearn how lambdas and tail recursion worked.