A classroom timer has one job: let every student see how long is left without having to ask. This one opens in the digital view, with large digits and a thick bar beneath them that shrinks as time runs down. The bar starts green and fades through orange to red as the minutes go, so even children who can't read a clock yet know when to wrap up.
Press Show on the board and the countdown fills the screen of whatever is connected to the projector or interactive whiteboard. The rest of the page disappears, so nothing competes with the clock, and the control bar fades out once the countdown starts. Press Escape or Exit full screen to get the desktop back.
It runs in the browser already open on the classroom computer. There is nothing to install and no account for the school to approve.