- You can't un-register.
- You can get audit notices, which can be computer selected or manual. You must reply to such notices in due time with income statements. If there is no income, just reply and tell them you're a student and don't have any income. You can attach your student card. Everything is done online, no need to go to office.
- If you've not filed tax returns (even nill) in due time, and file them late, FBR might open audit for those years, since late tax filing has a penalty of rs 40,000. FBR loves opening audits for late submissions due to this penalty (happened in 2018 on large scale). If you don't reply to audit notices, then there are further penalties, which can exceed rs 100,000.
I don't want to scare you, but tax rules are very predatory in Pakistan. Your best bet is to ignore these notices and start filing in future when you start making money (from that year).
Income Tax Ordinance:
182: Offences and penalty
(1) Where any person fails to furnish a return of income as required under section 114 within the due date.
Such person shall pay a penalty equal to 0.1% of the tax payable in respect of that tax year for each day of default subject to a maximum penalty of 50% of the tax payable provided that if the penalty worked out as aforesaid is less than forty thousand rupees or no tax is payable for that tax year such person shall pay a penalty of forty thousand rupees.