Really depends, I feel. If I were absent in some event I'd say something like "참석을 못해서 죄송합니다" which just means "Sorry I couldn't attend." 참석 (chamseok/attend) 못해서 (mot-he-seo/because [noun] did not do) If you want to be informal say "못와서 미안해" and the answer above with "죄송합니다. 교실에 없었어요" is technically also right, if you want to say explicitly that you weren't in the classroom. But supplementary details are key here so if you're not talking about a classroom but just, in general, my answer would fit better. Anyway, just saying 죄송합니다 suffices most of the time.

Hi. I am learning korean too) I think, answer may be like that: 죄송합니다. 교실에 없었어요 교실-classroom or 수업 - lesson