Do you know the usual procedure for sympathy cards, etc, at your school? If it's standard procedure that the cost is paid by the business (as at my old job, with the card from "Your coworkers at XXX company"), then I'd just send the card to be posted.
If it's usual to collect money and/or have everyone sign the sympathy card individually, you could send an e-mail if you liked - but I don't think it's necessary. Certainly I'd think that no one will judge you for not doing it.
I was so focused on answering the question that I forgot to say - I'm so sorry for your loss, Deb.