If you cannot access the store, or if the login button does not appear, you may need to change your security settings on your device or network.
Below are a list of sites that you should whitelist if you have filters in place. This will apply to your home router and your school's router.
If you are on an iPad, there are even more strict security settings. If you do not see a "login" button, you may need to change your security settings.
Go to: Settings->General->Restrictions
Then you can turn them all off, or select specific settings.