Visual studio cannot detect newly created class for asp.net "website"

I am trying to modify an existing .net "website" project by adding a new class to it. I added my class inside the App_Code folder (let's call this class "ClassA"), and tried to access it from the outside in an aspx.cs file. For some reason, right after I created the class, I can create an instance of it in my aspx.cs file without any warnings from Visual Studio (e.g., ClassA a = new ClassA()). But every time I rebuild the project, I get the following Error from visual studio (The type or namespace name 'ClassA' could not be found (are you missing a using directive or an assembly reference?)). What am I missing here?

Code for Class A -> App_Code/ClassA.cs

public class ClassA
{
  public string test;
}

Code for Test.aspx.cs

namespace A{
  public class Test : System.Web.UI.Page
  {
    protected void Page_Load(object sender, EventArgs e)
    {
      ClassA a = new ClassA(); // line with error
    }
  }
}

If I put everything in Test.aspx.cs I get no errors:

namespace A{
  public class ClassA
  {
    public string test;
  }

  public class Test : System.Web.UI.Page
  {
    protected void Page_Load(object sender, EventArgs e)
    {
      ClassA a = new ClassA(); // no error!
    }
  }
}