حَضَرَتْ إِحْدَى عَشْرَةَ طَالِبَةً — Eleven female students attended.
إِحْدَى عَشْرَةَ: compound numeral in the nominative position as subject; its two components are fixed on fatḥ, with an estimated fatḥah on the first component’s final alif maqṣūrah.
طَالِبَةً: singular accusative tamyīz.
عَشْرَةَ is the feminine-counted form. The numeral’s gender shape changes, but the general indeclinability rule still applies.