Hi Maite,
if you can access the site but the student can't it might be due to a certificate problem. Android is more strict than iOS regarding certificates, and old Android devices (4.x) might have problems connecting to sites using TLS1.2 certificates.
Regarding the original issue (site disappearing), in which devices and OS is it happening? Maybe the device is clearing the data of the app for some reason, or the app just fails to store the data (weird).
Kind regards,
Dani