bootstrap.js Accordion Collapse / Expand

The in class is just an indicator that a section is open. The Javascript module applies the same inline styles as .in does, so removing the class is insufficient.

You need to use the module’s API to programmatically interact with the accordion, via the .collapse() method:

$('.accordion-body').each(function(){
    if ($(this).hasClass('in')) {
        $(this).collapse('toggle');
    }
});

Or, you can condense this down to:

$('.accordion-body.in').collapse('toggle');

If you want only to collapse any open sections:

$('.accordion-body').collapse('hide');

If you want only to expanded any closed sections:

$('.accordion-body').collapse('show');