helper-functions.php

<?php

/**
 * Column Classes
 *
 * Adds "-first" classes when appropriate for clearing float
 * @see /assets/scss/partials/layout.scss
 *
 * @param array $classes, bootstrap-style classes, ex: array( 'col-lg-4', 'col-md-6' )
 * @param int $current, current post in loop
 * @param bool $join, whether to join classes (return string) or return array
 * @return array/string $classes
 */
function ea_column_class( $classes = array(), $current = false, $join = true ) {

	if( false === $current )
		return $classes;

	$columns = array( 2, 3, 4, 6 );
	foreach( $columns as $column ) {
		if( 0 == $current % $column ) {

			$col = 12 / $column;
			foreach( $classes as $class ) {
				if( false != strstr( $class, (string) $col ) && false == strstr( $class, '12' ) ) {
					$classes[] = str_replace( $col, 'first', $class );
				}
			}
		}
	}

	if( $join ) {
		return join( ' ', $classes );
	} else {
		return $classes;
	}
}

layout.scss

@include media(">=tablet", "<medium") {
    .col-sm-first {
        clear: both;
    }
}

@include media(">=medium", "<large") {
    .col-md-first {
        clear: both;
    }
}

@include media(">=large") {
    .col-lg-first {
        clear: both;
    }
}