Your students will only need to redeem your code one time to create a subscription for themselves.  After they have redeemed your code and created a profile (with a username/email address and password) they will not need to redeem your student code again until their account expires after one year.  They can use their subscription on multiple devices (including the iOS app).  All they need to do is just log in to their account using the login credentials they set up when they initially redeemed your code.  They can log into as many different devices as they want.