一个关于统计的update语句问题

现有一个student表，如下
ID Name Class
1 Susan Class1
2 Mary  Class1
3 Kate  Class2
我想要统计班级人数，将统计结果放在下表Class_Info中，
Class Number Monitor
Class1  2       **
Class2  1       ***
我用了这个语句，
UPDATE Class_Info, (SELECT Class, COUNT(Class) AS Number FROM student GROUP BY Class) AS a
SET Class_Info.Number = a.Number
WHERE Class_Info.Class = a.Class
但行不通啊，提示“operation must use an updateable query.”，大家快帮我看看错在哪了。不胜感激！