How do I unlink my iPad from my iPhone

Unlink how - or for what purpose? What, precisely, are you attempting to achieve?

Most Apple services (such as Contacts, Calendar, Messages etc.) use iCloud to synchronise data between devices. You can control behaviour from iPad and iPhone settings:

Settings > [Your Name / AppleID] > iCloud - enable or disable individual services as prefered

I am also wondering how to do this. I recently purchased a new iPad for school. Logging into my Apple account automatically transferred all the content on my iPhone onto my new iPad, which is fine, but I want to turn that feature off now as content from my iPad is also transferring onto my iPhone. I did need some of the content shared across the two devices, but how do I turn the feature off now while still keeping the current content that is already there?