Ah, I see. I'd say "Извините за задержку (с домашним заданием)". "...за опоздание" can work in theory here, but that word is kind of most readily associated with arriving late somehwere, so I strongly suggest that you refrain from using this word here.

P.S. I should have probably made this one out but to convey this idea quite clearly you need to "tweak" it like this:

Но это не я, кто опоздал. Это про моё домашнее задание.

xXHoax, you can read a little about "извиняюсь" here.

http://www.spb.aif.ru/culture/event/1460264

This -ся thing is all but relative as there are a lot of reflexive verbs that have lost this "refer back to the subject" vibe and adopted another meaning, so you don't need to worry about this much. After all this form of the word is on the verge of "making it", if you catch my drift.