SQL & PL/SQL

I have created scripts as follow

CREATE TABLE STUDENTS(
  STD_ID Number,
  STD_NAME Varchar2(25 ) CONSTRAINT SYS_C002716 NOT NULL,
  PHONE_NUMBER Char(20 ),
  FATHER_ADDRESS Varchar2(100 ),
  BASIC_GRADE Number(38,0),
  REGISTER_DATE Date,
  COMMENT_ON Char(200 ),
  STD_ADDRESS Varchar2(100 ),
  STD_RESPONSIBLE Varchar2(100 ),
  RESPONSIBLE_RELATION Varchar2(50 ),
  MOTHER_NAME Varchar2(30 ),
  STUDENT_TYPE Varchar2(10 ),
  YEAR Char(50 ),
  STD_NUMBER Number
)
/

-- Add keys for table STUDENTS

ALTER TABLE STUDENTS ADD CONSTRAINT SYS_C002717 PRIMARY KEY (STD_ID)
/

-- Table CLASSES

CREATE TABLE CLASSES(
  CLASS_NAME Char(40 ),
  CLSS_NUM Number(38,0) CONSTRAINT SYS_C009724 NOT NULL,
  LEV_ID Number(38,0)
)
/

-- Add keys for table CLASSES

ALTER TABLE CLASSES ADD CONSTRAINT KEY3 PRIMARY KEY (CLSS_NUM)
/
-- Table STUDENT_AND_CLASSES

CREATE TABLE STUDENT_AND_CLASSES(
  STD_ID Number NOT NULL,
  CLSS_NUM Number(38,0) NOT NULL
)
/

-- Add keys for table STUDENT_AND_CLASSES

ALTER TABLE STUDENT_AND_CLASSES ADD CONSTRAINT Key4 PRIMARY KEY (STD_ID,CLSS_NUM)
/

insert into students(std_id,std_name) values (1,'abk');
 insert into students(std_id,std_name) values (63,'saad');
 insert into classes(class_num,class_name)  values (3,'abk');
 insert into classes(class_num,class_name)  values (2,'osm');
 insert into classes(class_num,class_name)  values (1,'amr');

insert into student_and_classes values(1,1)
 
insert into student_and_classes values(2,63)

I have written a query :-

SELECT STD_NAME ,CLASS_NAME FROM STUDENTS S , CLASSES C
 WHERE
  STD_ID IN 
 (SELECT STD_ID FROM STUDENT_AND_CLASSES)
 AND
 CLSS_NUM IN
 (SELECT CLSS_NUM FROM STUDENT_AND_CLASSES)

STD_NAME                  CLASS_NAME
------------------------- ----------
abk                                amr
saad                              amr
abk                               OSM 
saad                              OSM 

I expected the query gives me student_name
in what class name and didn't expect to get frequency in output