添加学校表和年级表，修改班级表结构

Entities/Contracts/Grade.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Entities.Contracts
+{
+    [Table("grades")]
+    public class Grade
+    {
+        [Key]
+        [Column("grade_id")]
+        public int GradeId { get; set; }
+
+        [Column("school_id")]
+        public int SchoolId { get; set; }
+        public School School { get; set; }
+
+        [Column("grade_name")]
+        [MaxLength(20)]
+        public string GradeName { get; set; }
+
+        [Column("grade_level")]
+        public int GradeLevel { get; set; }
+
+        // Navigation Properties
+        public ICollection<Class> Classes { get; set; }
+
+        public Grade()
+        {
+            Classes = new HashSet<Class>();
+        }
+    }
+}