Essayez ceci
SELECT
MAX(CASE WHEN t.CLASS_LESSON = 'class1art' THEN t.ATTENDANTS ELSE NULL END) AS class1art,
MAX(CASE WHEN t.CLASS_LESSON = 'class1history' THEN t.ATTENDANTS ELSE NULL END) AS class1history,
MAX(CASE WHEN t.CLASS_LESSON = 'class2geography' THEN t.ATTENDANTS ELSE NULL END) AS class2geography
FROM
(
select
group_concat(distinct class, lesson) as class_lesson, count(*) as attendants
from
TableName
group by
class, lesson
) as t
