SELECT classes.id, classes.title, COUNT(pupils.id) 
FROM classes
LEFT JOIN pupils 
ON classes.id = pupils.id_class
GROUP BY classes.id;

mysql> create table classes (id int, title text);
Query OK, 0 rows affected (0.19 sec)

mysql> insert into classes (id,title)
    -> select 1,'5б' union
    -> select 2,'8' union
    -> select 3,'9А';
Query OK, 3 rows affected (0.07 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> create table pupils (id int, id_class int);
Query OK, 0 rows affected (0.20 sec)

mysql> insert into pupils (id,id_class)
    -> select 1,1 union
    -> select 2,2 union
    -> select 3,2 union
    -> select 4,2;
Query OK, 4 rows affected (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from classes; select * from pupils;
+------+-------+
| id   | title |
+------+-------+
|    1 | 5б    |
|    2 | 8     |
|    3 | 9А    |
+------+-------+
3 rows in set (0.01 sec)

+------+----------+
| id   | id_class |
+------+----------+
|    1 |        1 |
|    2 |        2 |
|    3 |        2 |
|    4 |        2 |
+------+----------+
4 rows in set (0.00 sec)

mysql> SELECT classes.id, classes.title, COUNT(pupils.id)
    -> FROM classes
    -> LEFT JOIN pupils
    -> ON classes.id = pupils.id_class
    -> GROUP BY classes.id;
+------+-------+------------------+
| id   | title | COUNT(pupils.id) |
+------+-------+------------------+
|    1 | 5б    |                1 |
|    2 | 8     |                3 |
|    3 | 9А    |                0 |
+------+-------+------------------+
3 rows in set (0.11 sec)