Does including Collections in Entities violate what an entity is supposed to be?

I am building a Web API using Dapper for .NET Core and trying to adhere to Clean Architecture principles. The API is consumed by an external Angular front-end.

I have repositories that use Dapper to retrieve data from the database, and this data then passes through a service to be mapped into a DTO for display to the user.

It is my understanding that an entity should be an exact representation of the database object, with no extra properties, and that I should use DTOs if I require some additional properties to show the user (or if I wish to obscure certain properties from the user too).

Suppose I have a DTO:

public class StudentDTO
{
  public Guid Id { get; set; }
  public string Name { get; set; }
  public List<Assignment> Assignments { get; set;}
}

and its corresponding Entity:

public class Student
{
  public Guid Id { get; set; }
  public string Name { get; set; }
}

With this model, should I want to get a student with all of their assignments, I'd need to have two repository calls, and do something like this in the service:

public StudentDTO GetById(Guid id) 
{
  var student = this.studentRepository.GetById(id);
  var assignments = this.assignmentRepository.GetByStudentId(id);
  return SomeMapperClass.Map(student, assignments);
}

But this seems inefficient and unnecessary. My question is, should I not just retrieve the Assignments when I get the student entity in the repository, using a JOIN? Or would this violate what an entity is supposed to be?

I apologise, I do realise this is a rather simple question, but I'd really like to know which method is the best approach, or if they both have their use cases