A userscript that adds a button to the assignment page on Google Classroom allowing you go back to your Work list. You'll need to install this script from Tampermonkey's "Utilities" page (copy the download URL, then paste it into TamperMonkey's URL import field), as Google Chrome doesn't like Gist.

GCBackToWork.user.js

// ==UserScript==
// @name        Classroom Back to Work
// @description Adds a button on the assignment page to go back to your work list, rather than the corresponding Class.
// @include     https://classroom.google.com/*
// @downloadURL https://gist.github.com/jrr6/2cfb984396d0e0d556ed2b117d5fe678/raw/GCBackToWork.user.js
// @updateURL   https://gist.github.com/jrr6/2cfb984396d0e0d556ed2b117d5fe678/raw/GCBackToWork.user.js
// @version     1.1.4
// @author      aaplmath
// ==/UserScript==

function addButton () {
    let button = document.querySelector('#b2w-button')
    let buttonExists = button !== null
    let isInAssignment = window.location.pathname.indexOf('/c') > -1 && window.location.pathname.indexOf('/a') > -1
    if (isInAssignment && !buttonExists) {
        let topBar = document.querySelector('.XIpEib.QRiHXd')
        if (topBar !== null) {
            let a = document.createElement('a')
            let div = document.createElement('div')
            let h1 = document.createElement('h1')
            div.setAttribute('id', 'b2w-button')
            a.setAttribute('href', '#')
            a.style.marginLeft = '1em'
            div.style.border = '1px solid rgba(0,0,0,0.87)'
            div.style.padding = '0.5em'
            div.style.position = 'absolute'
            div.style.top = '1em'
            h1.style.color = 'rgba(0,0,0,0.87)'
            h1.textContent = 'Back to Work'
            div.appendChild(h1)
            a.appendChild(div)
            topBar.appendChild(a)
            a.addEventListener('click', goBackToWork, false)
        } else {
            window.setTimeout(addButton, 500)
        }
    } else if (buttonExists && !isInAssignment) {
        button.parentNode.removeChild(button)
    }
}

function goBackToWork (e) {
    e.preventDefault()
    document.querySelector('a[href="/a/not-turned-in/all"]').click()
}

addButton()
const obs = new MutationObserver(function (mutations, observer) {
    if (mutations[0].addedNodes.length || mutations[0].removedNodes.length)
        addButton()
})
obs.observe(document.body, {
    childList: true,
    subtree: true
})