给予列命名：

ResultSet rs= stmt.executeQuery("select count(name) AS count_name from db.persons where school ='"+sch+"'");
if (rs.next()) {
    int count= rs.getInt("count_name");
}

你也可以传递列的索引号（如果你不想修改你的查询），它是从1开始的。

ResultSet rs= stmt.executeQuery("select count(name) from db.persons where school ='"+sch+"'");
if (rs.next()) {
    int count= rs.getInt(1);
}

除此之外，如果使用PreparedStatement来执行查询会更好，它比普通的Statement有很多优点，如下所述：Difference between Statement and PreparedStatement。您的代码如下所示：

String sql = "select count(name) AS count_name from db.persons where school = ?";
PreparedStatement pstmt = con.prepareStatement(sql);
pstmt.setString(1, sch);
ResultSet rs = pstmt.executeQuery();
if (rs.next()) {
    int count = rs.getInt("count_name");
}