You may want to rethink your class structure and variable names. I'm not sure what MyList is supposed to represent, a student or a classroom. You can solve the problem at hand with a Select statement, though:
List<MyList> ClassRooms = GetClassRooms(id)
.Select(classroom => GetStudentProfile(id, classroom.ID));
return ClassRooms;
This will take each class in the list of classrooms, call GetStudentProfile on it, and add that to the list.