[susan53]

(only one type of English class)
A)their English calsses
B)their English class



Notice there are no articles here, so the question doesn't really fit your heading. Their is a possessive adjective. However :

Antonio and Alessandro are twins. They both study English in a private school at 3pm on Fridays. They are in the same class.
At 3.30 someone says to their mother Where are the twins? She replies : They're at their English class. = two boys (they/their), one class (singular).

Mariella and Marta are twins. They both study English in a private school at 3pm on Fridays. They are in different classes.
At 3.30 someone says to their mother Where are the twins? She replies : They're at their English classes. = two girls (they/their), two classes (plural).