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

Entities/Contracts/School.cs

@@ -9,8 +9,8 @@ namespace Entities.Contracts
     public class School
     {
         [Key]
-        [Column("school_id")]
-        public int SchoolId { get; set; }
+        [Column("id")]
+        public Guid Id { get; set; }
 
         [Column("school_name")]
         [MaxLength(50)]
@@ -23,11 +23,12 @@ namespace Entities.Contracts
         [Column("create_time")]
         public DateTime CreateTime { get; set; }
 
-        // Navigation Properties
-        public ICollection<Grade> Grades { get; set; }
+        [InverseProperty(nameof(Grade.School))]
+        public virtual ICollection<Grade> Grades { get; set; }
 
         public School()
         {
+            Id = Guid.NewGuid();
             Grades = new HashSet<Grade>();
             CreateTime = DateTime.Now;
         }