वृद्ध संज्ञा is attached to the whole word (the group not पद necessarily) whenever वृद्धि is applicable to one amongst many अच्s. In शालीय, वृद्धि of आ in शा is considered वृद्धि of शाला (Note - शाला ङ्इ (अण्) ). The "छ" action is therefore applied to the words(group) which had achieved the said वृद्धि.