If it was me, I would just expand your pool of IP address space for the devices that are attaching. Note that you do expose yourself to a possible DHCP exhaustion attack using the approach you are using (and you are in a school ...). With a DHCP exhaustion attach you can download existing attack tools, and all they do is send DHCP requests using different MAC addresses until the DHCP server has no IP address space left to give out to real clients. The second approach I would use is to just use a NAT mode SSID. With 16 million IP addresses it makes a DHCP starvation attack improbable. With the hashing method that Meraki uses with a NAT mode SSID to generate DHCP client addresses - it is probably impossible.
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