First you will need to add the school calendar to your Google calendar:
On your computer, at the bottom right of the Calendar (to the left), click on the button “+ Google Calendar”.
You will be asked if you would like to add this Google Calendar. Click Yes.
To add the calendar to your iPhone:
Set your Google account settings by going to Settings>> Mail, Contacts, Calendars, on your iPhone.
After validating your account, open the browser and go to http://google.com/calendar/iphoneselect on your iPhone. You may be prompted to login. Login with your Google account information.
After you are logged in you will see a screen that lists your calendars.
Put a check in the boxes in front of the calendars you want to sync to your iPhone and touch save at the bottom right.
The next screen prompts you to open your iPhone calendar app and view your calendars. It is that simple.