I think there are two questions in your request: one about building an attendance app, the other about monetizing your app by using a SaaS business model.
Building an attendance app
An attendance app is a nice use case for Glide. You could use Google Sheets as a data source as Uzo suggested. Glide Tables, Airtable or Excel would be other possible data sources. Which you choose is a matter of personal preference, of performance (Glide Tables is excellent), of the need for certain features that maybe only an external source might provide. Google Sheets for example might be a good choice if you are proficient in Apps Script, Airtable might be a good choice to leverage the native automations of Airtable, Glide Tables would be a good choice for overall performance and simplicity.
SaaS business model
Natively in Glide, there is no system where you the builder could build an app and sell it against monthly recurring revenue. Other ideas however: build and sell a template, sell a one-time build for a client, sell ongoing maintenance for a client.
The following thread is old and some of the ideas are now out of date, but maybe it will give you ideas.