fn.GROUP_CONCAT should support ORDER BY
Try this:

```python
fn.GROUP_CONCAT(Clause(
    SQL('Distinct'),
    MyModel.test_score,
    SQL('ORDER BY'),
    MyModel.test_score,
    SQL('DESC SEPARATOR \' \'')))
```

Not very clean but that should hopefully help you get the SQL you need.
From [MySQL Docs](http://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_group-concat), ```GROUP_CONCAT``` supports ```ORDER BY```

```
mysql> SELECT student_name,
    ->     GROUP_CONCAT(DISTINCT test_score
    ->               ORDER BY test_score DESC SEPARATOR ' ')
    ->     FROM student
    ->     GROUP BY student_name;
```

