See the following Apple articles:
Use cellular data on your iPhone or iPad
Use cellular data on your iPhone or iPad - Apple Support
View or change cellular data settings on iPhone
View or change cellular data settings on iPhone – Apple Support (UK)
(note that i go eight hours to school, which has no wifi lol)
Try to use Public Wi-Fi if possible, see this Apple article: Use captive Wi-Fi networks on your iPhone, iPad or iPod touch -> Use captive Wi-Fi networks on your iPhone or iPad – Apple Support (UK)
Nowadays, Public Wi-Fi is offered for free, in big cities, shopping centers, in Public Transport (bus & train), airport, restaurants and others. Perhaps they also have public Wi-Fi at your school (?)
That depends on where you live.