If the disciple is related physically to the Acharyan, then theetu should be observed as per the usual norms applicable to a physical relative.
Even if the disciple is not physically related but
If Acaryan is a Gruhasthan (Family man), one has to observe one-day theetu as per the practice.
If the Acarya happens to be a yati or saint then there is no theetu for the disciple.