Injecting a javascript code to all requested web pages by a desktop application

I work in a community school and I have been requested to censor some words in all requested web pages (page content-html level). If I could inject a javascript code I can do that with a simple javascript code.

My question is can I inject that javascript code to every requested web page with a desktop program? If yes, can you recommend me some topics so I can start research?

I'm a web developer. I could also do that with browser extensions but managing browser extensions are hard for all browsers that's why i am looking a solution at system level.