Hangouts is a chat and video conference facility that is integrated into Google Apps and is free to all who have a Google+ account or a Google Apps account.  A standard user can join a video call with up to 10 participants, and a Google Apps for Education user can have up to 15 people as part of the call.  It is available on a variety of devices and all your data is synced across these.

Some ideas for using Google+ Hangouts in your school:

Some Ideas and Examples for Using Google+ Hangouts in School

Getting Started with Google+ Hangouts

Google+: Explore Hangouts

How to use Hangouts - Google Gooru

Google+: Fraser Cain's Virtual Star Party

Hangouts: Conversations that last, with the people you love