I think you must log in to the Powerschool system to install the plugin before you can connect to the API.
The plugin should look something like this...
<?xml version="1.0" encoding="UTF-8"?>
<plugin xmlns="http://plugin.powerschool.pearson.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation='http://plugin.powerschool.pearson.com plugin.xsd'
name="(Your Product Name)"
version="(Your Product Version)"
description="The plugin for PowerSchool integration with (Your Product Name)">
<oauth></oauth>
<publisher name="(Your Company Name)">
<contact email="(Your Email Address)" />
</publisher>
<access_request>
<field table="CodeSet" field="description" access="ViewOnly" />
<field table="CodeSet" field="codetype" access="ViewOnly" />
<field table="CodeSet" field="codesetid" access="ViewOnly" />
<field table="CodeSet" field="code" access="ViewOnly" />
<field table="STUDENTS" field="DCID" access="ViewOnly" />
<field table="STUDENTS" field="ID" access="ViewOnly" />
<field table="STUDENTS" field="first_name" access="ViewOnly" />
<field table="STUDENTS" field="last_name" access="ViewOnly" />
<field table="STUDENTS" field="grade_level" access="ViewOnly" />
<field table="TRANSPORTATION" field="DCID" access="FullAccess" />
<field table="TRANSPORTATION" field="ID" access="FullAccess" />
<field table="TRANSPORTATION" field="StudentId" access="FullAccess" />
<field table="TRANSPORTATION" field="Description" access="FullAccess" />
</access_request>
</plugin>
There are various places where you need to put your own information, like company name, product name, etc...
Then, you will need to put the tables and columns you want/need access to in the access_request node. Note, you must specify the type of access you require, such as ViewOnly or FullAccess.
This is a security measure. The Powerschool administrator for the school you are dealing with can then decide whether to allow the plugin or not. Once the plugin is installed, the Powerschool administrator should tell you the ClientId and the client secret to use for OAuth authentication.