添加学校表和年级表，修改班级表结构

Entities/Contracts/School.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Entities.Contracts
+{
+    [Table("schools")]
+    public class School
+    {
+        [Key]
+        [Column("school_id")]
+        public int SchoolId { get; set; }
+
+        [Column("school_name")]
+        [MaxLength(50)]
+        public string SchoolName { get; set; }
+
+        [Column("address")]
+        [MaxLength(100)]
+        public string Address { get; set; }
+
+        [Column("create_time")]
+        public DateTime CreateTime { get; set; }
+
+        // Navigation Properties
+        public ICollection<Grade> Grades { get; set; }
+
+        public School()
+        {
+            Grades = new HashSet<Grade>();
+            CreateTime = DateTime.Now;
+        }
+    }
+}