@Peter_Edusuite - thanks for clarifying. From your initial post, I thought you were asking specificly about building a Portal app.
>> If I crease an app using my A3 licence, can the other school users use the app I have produced without needing an additional licence (beyond the A3) or do they count as additional users and in that case our school will be subject to the additional power app specific licence fees?
In answer to this, if you create a "Canvas" app that uses SharePoint or standard connectors, all other users with an A3 license can use your app without additional licensing.
An A3 license will not cover the use of a "Model Driven App", nor will it cover a Portal app. For this, you will need additional licensing.
I'm not sure about specific educational pricing. I have worked for charities who have secured preferential licensing, so I know that schemes exist that are not openly advertised. Hopefully, someone else here may know the answer to this.