My School tax was due on January 31st but I mailed it only a few days late with a postmark of Feb 5th and a late fee was assessed. Is this legal?

Tax payments are deemed to have been paid as of the date of the postmark.  If a payment is postmarked after the due date, interest and penalties will accrue.  If the envelope arrives at the Tax Office without a postmark, unfortunately it is deemed paid on the date of its arrival.