こうかな？

CREATE TABLE class (
id int NOT NULL PRIMARY KEY,
name text NOT NULL
);
INSERT INTO class(id,name) VALUES (1,'A組');
INSERT INTO class(id,name) VALUES (2,'B組');

CREATE TABLE person (
id int NOT NULL REFERENCES class(id),
number int NOT NULL,
name text NOT NULL,
PRIMARY KEY (id, number)
);
INSERT INTO person(id,number,name) VALUES (1,10,'Cさん');
INSERT INTO person(id,number,name) VALUES (2,10,'Dくん');
INSERT INTO person(id,number,name) VALUES (2,11,'Eくん');

CREATE TABLE info (
id int NOT NULL,
number int NOT NULL,
info text NOT NULL,
FOREIGN KEY (id, number) REFERENCES person (id, number)
);

INSERT INTO info(id,number,info) VALUES (1,10,'Cさんの情報1');
INSERT INTO info(id,number,info) VALUES (1,10,'Cさんの情報2');
INSERT INTO info(id,number,info) VALUES (2,10,'Dくんの情報1');
INSERT INTO info(id,number,info) VALUES (2,11,'Eくんの情報1');
/* INSERT INTO info(id,number,info) VALUES (1,11,'外部キー制約でエラー'); */