Consultation to calculate the number of female and male students

I translate the Aluno table of your model to PL/PgSQL :

CREATE TABLE Aluno
(
  idaluno BIGINT PRIMARY KEY,
  nome TEXT NOT NULL,
  cpf VARCHAR(11) NOT NULL,
  rg VARCHAR(14) NOT NULL,
  sexo VARCHAR(1) NOT NULL,
  data_nascimento DATE NOT NULL
); 

Registering Students:

INSERT INTO Aluno ( idaluno, nome, cpf, rg, sexo, data_nascimento ) VALUES
( 100, 'JOAO', '11122233300', '1234567', 'M', '1980-03-10' ),
( 200, 'JESUS', '99988877766', '45678', 'M', '1970-06-17' ),
( 300, 'MARIA', '44455566677', '09876', 'F', '1953-04-30' ),
( 400, 'MADALENA', '33399955511', '345677', 'F', '1965-10-23' ),
( 500, 'JOSE', '00987243643', '8181818', 'M', '1988-03-03' );

Solution # 1: With GROUP BY and count() :

SELECT
  sexo,
  COUNT(1)
FROM
  Aluno
GROUP BY
  sexo;

Output:

| sexo | count |
|------|-------|
|    F |     2 |
|    M |     3 |

Solution # 2: With sum() and without GROUP BY :

SELECT
   SUM(CASE WHEN sexo = 'M' THEN 1 ELSE 0 END) AS qtd_masculino,
   SUM(CASE WHEN sexo = 'F' THEN 1 ELSE 0 END) AS qtd_feminino,
   SUM(1) AS qtd_total
FROM
  Aluno;

Output:

| qtd_masculino | qtd_feminino | qtd_total |
|---------------|--------------|-----------|
|             3 |            2 |         5 |

SQLFiddle: link