GROUP BY musí přijít před ORDER BY:
SELECT SUM(g.points)
FROM GRADES g
WHERE g.date < 'thedate'
GROUP BY g.assignmentid
ORDER BY g.date DESC
GROUP BY musí přijít před ORDER BY:
SELECT SUM(g.points)
FROM GRADES g
WHERE g.date < 'thedate'
GROUP BY g.assignmentid
ORDER BY g.date DESC