SSL vs ISP

I have the Cloudflare SSL up and running on a bunch of sites but for some reason, I have 1 site with an issue.

It’s for a school and it works everywhere except in school, frustrating I know. If you are on the internal school network it wants to load the old GoDaddy SSL, which no longer exists, I thought that it was a caching issue so I waited a few days but the issue hasn’t gone after a month or so.

I have removed all of the old accounts and any references to the old SSL but the ISP for local schools keeps batting it back to me but with the site literally working everywhere else I can’t think of what it can be.

https://www.langstone-jun.portsmouth.sch.uk

Could be cached DNS entries, or probably a proxy? Did you check that?