@Madison2  Hello, 

If possible, it is recommended to run an Ethernet cable between router and range extender and use it in AP Mode, in this way RE could work more stable. 

Here is the instruction to set up an Access Point for RE:

(via Web UI)https://www.tp-link.com/support/faq/1401/
( via Tether APP) https://www.tp-link.com/support/faq/2852/

Factory reset Re before you set it up: with RE power on use a pin to press the reset button until all the Leds flash once

If you could not run a cable between router and range extender,  please plug in range extender next to router during set up, how about the Leds status after the setup set up?

Here is a guide to troubleshoot RE installation step by step, kindly give it a try and let us know if it works for you,
Has connected to TP-Link extender but no internet, what can I do?
https://community.tp-link.com/en/home/forum/topic/198516

May I know the model of router,  does your school use a Portal wifi, or need any authentication when a new device connected to it?