Get Data from website and math


I wanna build an app that will have a log in page to connect to my school website and goes to the grades link and get those grades and estimate the final grade for the student know how well it should take to the next tests. I have know idea how to connect to the website from my form and how to “extract” this data and make the math.
Just to make it clear I don’t have access to the database. I should happen with the student login to the website.

Can anyone help?

Thanks !