DAL-BLL and List question

Question

I have a DAL-BLL question. Basically, I have a some information that im splitting into two tables. Eg. I have a GradeMetadata table and a Students Table. The students table consists of all the students in a school, but it also has a field for the GradeMetada ID, so that way I can track of which kid is in which Grade.

now in my DAL, I have a

GradeDetails class and a StudentDetails class.

namespace DAL
{
GradeDetails has the following properties:
    id, gradeName, and a List<studentDetails>

StudentDetails has the following properties    
     id, studentName, age    (in my insert logic I also populate a column which ahs the gradeDetails id.)
}


now in my BLL I inititially had a Grade class as follows:  

using DAL;
namespace BLL {
    Grade has the following properties
    Id, GradeName, List<studentDetails>
}

During normal usage, I would only want to use    using BLL,
but what I came to realise is that if I have to create a studentDetails object to pass into Grade..I cant do that cause there is no definition for a studentDetails in BLL.

So I created a StudentDetails class inside the Grades class like so:
namespace BLL{
    class Grade 
   {
        Id, GradeName, List<studentDetails>

        class Students
            Id,  StudentName, Age
   }

}


but I keep getting this error that in my insertlogic cannot convert from <BLL.Grade.Students> to <DAL.StudentDetails>

this seems like a valid error message because the compiler doesnt know that the List<studentDetails> and List<Grade.Students> has the identical structure...so how would I get around this?

thanks

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