You can access your class sites using your usual school network login details. If you access Schoology via the school portal then there is an automatic sign in.
If you access Schoology from this website using this page or directly by bookmarking http://perse.schoology.com then you will login with your usual school network username/password.
If you download the Apple Schoology app on your iphone/ipod/ipad then to use the app you must first set a schoology password by logging into Schoology from a computer and adjusting your account settings : schoology password. Then from the app you can log in with your school email address and the password you have created.
If you do create a Schoology password then that is also valid directly on the public Schoology page www.schoology.com