The IUSR account won't work if you have more than one web server. IUSR can't access network folders.
Assuming you school is running Active Directory, you need to use a \domain\moodleuser AD service account in IIS. And you need to give that \domain\moodleuser account full read/write access to the network location where you've created your Moodledata folder. And you need to run your Moodle IIS site (or virtual directory / application) under that \domain\moodleuser service account.