create table grades
(stuId bigint identity(11111111,1) primary key,
stuName varchar(20) not null,
classId varchar(8) primary key,
className varchar(20) not null,
semester varchar(20) not null,
pscores int,
escores int,
foreign key classIdreferences classes(classId)--这样就好了
foreign key grades(classId) references classes(classId))
go

貌似你写的有错，应该是classId 的外键吧

------解决方案--------------------

create table grades
(stuId bigint identity(11111111,1) primary key,
stuName varchar(20) not null,
classId varchar(8) primary key,
className varchar(20) not null,
semester varchar(20) not null,
pscores int,
escores int,
foreign key (stuId) references classes(classId)--,
--foreign key (classId) references classes(classId))
go

不要前面的表名字，而且你这一个表怎么能有两个外键呢？

------解决方案--------------------
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引用:
create table grades
(stuId bigint identity(11111111,1) primary key,
stuName varchar(20) not null,
classId varchar(8) primary key,
className varchar(20) not null,
semester varchar(20)……