This morning I was doing some searching for sms-based online polling (clickers are too expensive and oh-so-20th century). PollEverywhere looks great, but I also happened to stumble on this notification service called SendGM. It does basic polling, but its real forte is notification.
You enter contact information into your account, and from there you can assign the contacts to any combination of groups. The real power comes in the types of notifications that SendGM can provide. It will send any combination of email, sms, and text-to-voice notifications based on the contact info for each person. You can also request replies from the contacts (hence the polling feature).
The free account handles up to 25 users, but the next level (100) is only $79 per year. Not bad. I like that it is a more private, controlled, and flexible system than Twitter. This could be the ultimate school group notification system, especially for travel.