Please notice that the phrase "arrives late" can also mean "מאחר".
Any variation which includes "מאחר" is acceptable too.
For example, this translation is correct even though Doulingo doesn't accept it:
"כל תלמיד (או סטודנט) אשר מאחר, לא מקבל ציון."
What is formal about this?
I guess that the formal part is the use of ״ה״ instead of ״ש״ in ״המגיע״ (as opposed to ״שמגיע״).
Makes sense lol