Is there a way to prevent a client side JS injection into my website?

I have my website, which my school has recently blocked. I found how to get around it by deleting the filter agents functions and variables (That are injected in JS) before my website loads.
However, I was wondering if there was a way to prevent the injecting JS on my website in the first place. I am using GitHub pages.

I have tried resetting the DOM tree, but I kept getting errors that I could not do this, I don’t have the error message right now, but I’ll get it soon. (Sorry about that)

flushFile = document.createElement("script");
    flushFile.setAttribute("src", "flush.js");
    document.head.appendChild(flushFile); //Loads flush.js into the DOM tree, overwriting all functions and variables of the filter agent, in browser memory to null

I am aware that most XSS attacks are on server side, but I am not worrying about that (for now). I just need to fix something like client side. If I disable all JS, yes that defeats this, but my nav bar would break, and lots of other things on my website. (https://ranchodvt.github.io/Comp-V3/index.html)

Here’s my code of deleting all the filter agents variables and funtions:

document.addEventListener('DOMContentLoaded', () => {
    
    function timeout(ms) {
        return new Promise(resolve => setTimeout(resolve, ms));
    }

    async function NotBlockingMe() {
        await timeout(75);
        console.clear();
        console.log("=====");

        // Functions
        var getLoaderPolicy = function () { }    // no-op function;
        var loadES6 = function () { }    // no-op function;
        var isYoutube = function () { }    // no-op function;
        var checkCurrentSite = function () { }    // no-op function;
        var getHardBlockPolicy = function () { }    // no-op function;
        var hardBlock = function () { }    // no-op function;
        var stopVideo = function () { }    // no-op function;
        var updateLocation = function () { }    // no-op function;

        // Variables
        var hardBlockPolicy = null;
        var prevURL = null;

        console.log("=====");

        // re-assign
        window.isYoutube = function () { }    // no-op function
        window.loadES6 = function () { }    // no-op function
        window.checkCurrentSite = function () { }    // no-op function
        window.getHardBlockPolicy = function () { }    // no-op function
        window.hardBlock = function () { }    // no-op function
        window.stopVideo = function () { }    // no-op function
        window.updateLocation = function () { }    // no-op function
        window.initFlagScanning = function () { }    // no-op function
        window.getLoaderPolicy = function () { }    // no-op function
        window.loaderPolicy = function () { }    // no-op function

        console.log("Just incase, Functions deleted again.");

        console.log("=====");
    }
NotBlockingMe()
// Other website code...
}

If I forgot something obvious, or you have a question, please don’t hesitate to ask.
I’m new to website design.