1)  For iPhones 

Go into the Settings app 
Touch "Accounts & Passwords" 
On the Accounts & Passwords screen, touch "Add Account" 
On the Add Account screen, touch "Other" 
Under Calendars, touch "Add Subscribed Calendar" 
For the Server value, enter the URL for the teacher calendar: 
https://www.charmsoffice.com/charms/calsynct.asp?s=RiverviewBand 
Touch "Next" in the upper right corner. 
It will verify the server then show a subscription page. Change the description if you like. 
Touch "Save" to save the subscription.

2)  For Android Phones 

To sync with Android devices, you will need to use your Google account and have a Google calendar created. 

In your Google calendar on a real computer: 

Under "Other Calendars" click the Add link. 
Select "Add by URL" 
Enter the following URL: 
https://www.charmsoffice.com/charms/calsynct.asp?s=RiverviewBand 
Click "Add Calendar" 
Google will begin to sync your calendar, and it will appear on the screen. 
Click "Settings" 
Change the long name of the new calendar to something you will remember such as "Charms Calendar" 
Your phone should automatically be set to sync with your Google calendar. 
On your phone: Click on "Calendar", then bring up the settings for the calendar. Click on Calendar Sync. You should see your google account listed already. Make sure Auto-Sync is checked.