Try this way:
woocommerce_form_field( 'student_name', array(
'type' => 'select',
'class' => array('my-field-class','form-row-wide','ram_select2'),
'label' => __('Student'),
'placeholder' => __('Student Name'),
'required' => true,
'options' => $patients,
), $checkout->get_value( 'student_name' ));