Need More than 255 Devices on School Network

Need some advice please. I work for a school and because of covid19 the schools network has become device hungry due to online learning. We have a standard adsl router which brodcasts dhcp addresses. A server running Windows Server 2012 R2 and Unifi system supporting 17 Access Points all over the school. Devices can’t connect as 255 is max. What is the best method to enable more than 255 devices to connect to the network?