EduSync allows teachers to sync the calendar interface on EduSync's site to their Google Calendar AND to Google Classroom. So instead of teachers trying to maintain a Google Calendar and also post announcements and assignments to Classroom, it can be done all at once.
On the EduSync site teachers can easily add their classes and units and access the calendar. The interface is very easy to use with lots of options for teachers to identify the type of event, add dates and standards, and give instructions and objectives. It's very well thought out and you can tell it was designed by teachers, PLUS it's free forever.
To add an event simply click on the day on the calendar, adjust the details for your event and save. The beauty of it for me is the Google Classroom button on the bottom left that allows you to instantly add the event as an assignment or announcement to your Google Classroom. No more dancing between programs and cutting and pasting.
There are a few steps to pay attention to in order to sync your EduSync calendar with your Google tools, but it's as simple as ticking a few boxes in Settings.
They also have a mobile app called EddyCal that is slated to come out soon that students will be able to use as a calendar. You can request to be a part of their pilot for EddyCall here.
Great stuff. Here is a video about the product.