我有下表：

+------------+-------+
| SchoolName | Marks |
+------------+-------+
| A          |    71 |
| A          |    71 |
| A          |    71 |
| B          |   254 |
| B          |   135 |
| B          |   453 |
| B          |   153 |
| C          |   453 |
| C          |   344 |
| C          |   223 |
| B          |   453 |
| D          |   300 |
| D          |   167 |
+------------+-------+

以下是按学校名称分组的平均分数：

+------------+------------+
| SchoolName | avg(Marks) |
+------------+------------+
| A          |    71.0000 |
| B          |   289.6000 |
| C          |   340.0000 |
| D          |   233.5000 |
+------------+------------+

https://www.db-fiddle.com/f/5t7N3Vx8FSQmwUJgKLqjfK/9
然而，我想计算的不是平均数，而是按学校名称分组的分数中位数。
我在用，

SELECT AVG(dd.Marks) as median_val
FROM (
SELECT d.Marks, @rownum:=@rownum+1 as `row_number`, @total_rows:=@rownum
  FROM tablename d, (SELECT @rownum:=0) r
  WHERE d.Marks is NOT NULL
  ORDER BY d.Marks
) as dd
WHERE dd.row_number IN ( FLOOR((@total_rows+1)/2), FLOOR((@total_rows+2)/2) );

来计算整个"分数"列的平均值，但我不知道如何针对每个学校分别进行计算。