At home I've had several times when my home wifi network was not detected by my Roku. I've always been able to correct this by restarting my router and then my Roku. But I suspect you have no way to restart the school's router.
See if this will get it for you:
Settings > System > Advanced system settings > Network connection reset
This should clear out all stored network information and if you're lucky will remove whatever is blocking your Roku from seeing you school's wifi.
Since you say you have an Express, there is one other possibility that comes to mind. Express and Premiere are budget models have single band wifi radios operating in the 2.4 GHz band. If, since last year, the school has switched its wifi to only the 5 GHz band, your Express can't work with it. Most (all?) router systems supporting 5 GHz also support 2.4 GHz, although this may be turned off -- you might want to check with the system's administrators about this.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Do not ask me support questions via private message -- post them publicly to the Community where others may benefit as well.
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