重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/DTO/Grade/GradeResponseDto.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class GradeResponseDto
+    {
+        public int Id { get; set; }
+        public int SchoolId { get; set; }
+        public string SchoolName { get; set; }
+        public string GradeName { get; set; }
+        public int GradeLevel { get; set; }
+
+        // Navigation properties for response
+        public ICollection<ClassDto> Responses { get; set; }
+    }
+}