Is it obligatory to perform the Sunnah Rawatib before the Dhuhr prayer, or can it be made up later, especially when joining the congregation after the time for performing it before the prayer has passed?

If you enter the mosque and find the students praying the obligatory prayer, join them in prayer and do not busy yourself with the Ratibah (supererogatory prayer accompanying an obligatory one); this is based on the Prophet's (peace be upon him) saying: "When the iqamah is called for prayer, there is no prayer except the prescribed (obligatory) one." After the prayer ends, you may perform the Ratibah that precedes it.