Small script that adds a button to the Piazza toolbar to mark all posts in the currently selected class as read.

markPiazzaAsRead.js

// Button to mark all posts as read
function makeButton() {
    let button = document.createElement("button");
    button.setAttribute("id", "new_post_button");
    button.setAttribute("class", "btn-primary left btn-small");
    button.addEventListener("click", _ => {
        // Clear unread posts and reload page
        document.querySelectorAll(".unread").forEach(b => b.click());
        window.location.reload();
    });

    let text = document.createElement("i");
    text.setAttribute("class", "icon-check icon-white");
    button.appendChild(text);

    return button;
}

let toolbar = document.querySelector("#feed_search");
toolbar.insertBefore(makeButton(), toolbar.children[0]);

I recommend adding this as a userscript that runs on page load for ease of use.