In my school, all HTTP traffic has to be filtered through a proxy
server, say proxy.school.edu:8080. However, it appears that my school
network is linked to the Internet via a leased line from a local ISP
which has its own proxy server, say proxy.isp.com:8080.

I experimented with using both servers as my HTTP proxy setting in my
web browser and found that both can work.

I hope to know exactly which HTTP proxy am I using anyway?

I'm surprised that I can use a proxy server outside of my school
network over at the ISP side and I wonder how my HTTP traffic managed
to get itself routed out of the school's network directly to the ISP
proxy, which is supposed to be considered outside of my school's
network and hence should be blocked by some firewall.

Could anyone kindly enlighten me on why this can happen? Tnanks.