I often send a card. I also try to go to the wake or funeral if I can. But, I'm in the south, and that's just what we do here. Also, I think you're an online teacher, so a card would be more appropriate. I'm not religious, so I usually get a non-religious card and do that. I write, "I'm so sorry for your loss. Please let me know if I can help in any way." Once, with a kid I was really close to, I included a gift card for a restaurant they go to a lot, and a note about how I know traditionally I should bring food, but I don't cook and so they would not want to eat what I would cook.