MySQL 怎么删除索引?我试了试怎么不行?

这是表的创建属性:

CREATE TABLE `c200402` (
  `id` tinyint(3) unsigned zerofill NOT NULL AUTO_INCREMENT,
  `学籍编号` char(10) DEFAULT NULL,
  `姓名` char(15) NOT NULL,
  `性别` enum('男','女') DEFAULT '男',
  `档案` char(20) DEFAULT '沂水县实验中学',
  `职务` set('班长','副班长','学习委员','文艺委员','体育委员','团支部书记') DEFAULT NULL,
  `time` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `学籍编号` (`学籍编号`),
  KEY `姓名索引` (`姓名`),
  KEY `学籍索引` (`学籍编号`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;

我的命令: 三个都试了

drop index 学籍索引;
drop index '学籍索引';
drop index "学籍索引";

错误信息:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near '' at
line 1

请问该怎么做?

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drop index 索引名 on 表名