Although most schools probably have a robust email contact system in place by now (we use Mail Chimp), you might not have individual email lists for each teacher and classroom. Most teachers just manually collect email addresses at their “Meet the Teacher” info night and set up their own system.
Well Remind101 ($Free) is a nice little app that will help you safely and securely contact parents or students.
The app itself is very easy to navigate. You can set up and name multiple classes, see the instructions for adding subscribers, see a list of your subscribers (and whether they use text, email, or both), and see a list of your sent messages. You can then send messages from within the app itself. Your messages will be instantly delivered to all the people that have subscribed. You can also contact a subset of the list instead of all subscribers.
The best thing about this app is that it protects parents' and students' privacy. By going through the steps to sign up, the subscribers at no point share their email address or phone number with the teacher. The app keeps it all secret and invisible. This means that subscribers can feel safe knowing that the teacher cannot send communications to them from outside the app. If they want to cut off communication, they can easily unsubscribe themselves from the list. Parents can feel safe subscribing their children to their teacher's list, as the teacher won't know the students' contact info. It is safe for all parties involved.
The only hard part, as a teacher, is making sure people actually subscribe. Not all parents show up to info fairs and conferences. For some, you might have to know their email in order to send them an invitation in the first place. Other parents don't want to be bothered by ANY classroom communications. It will take a bit of hunting and a bit of luck to get 100% participation. But once it is set, you can send out reminders, calendar updates, messages, and anything else by just loading the app, typing it up, and hitting send. Quick and easy. And free.