List, virtual and Entity Framework

In all examples I have found about entity framework, they always use ICollection for object collections.

public partial class Standard
{
    public Standard()
    {
        this.Students = new HashSet<Student>();
        this.Teachers = new HashSet<Teacher>();
    }

    public int StandardId { get; set; }
    public string StandardName { get; set; }
    public string Description { get; set; }

    public virtual ICollection<Student> Students { get; set; }
    public virtual ICollection<Teacher> Teachers { get; set; }
}

public partial class Teacher
{
    public Teacher()
    {
        this.Courses = new HashSet<Course>();
    }

    public int TeacherId { get; set; }
    public string TeacherName { get; set; }
    public Nullable<int> StandardId { get; set; }
    public Nullable<int> TeacherType { get; set; }

    public virtual ICollection<Course> Courses { get; set; }

    public virtual Standard Standard { get; set; }
}

Questions:

1 Can not work directly with the concrete type, such as List?

public partial class Standard
{
    public int StandardId { get; set; }
    public string StandardName { get; set; }
    public string Description { get; set; }

    public virtual List<Student> Students { get; set; }
    public virtual List<Teacher> Teachers { get; set; }
}

2 I also read that EF does not work with collections of primitive types. What to do in this case?

3 Because the property needs to be virtual, does the EF make any changes to it?