Hello,
Check why a guest create an account via "create one" option (subsite from multisite) https://prnt.sc/13d1okm the system assign Participan Role instead of Student Role.
But if he create an account via Gmail the system asign student role.
Hi
Please check what is the default role set for your subsite.
It's present in settings>>default role
for multisite-
Go to "My Sites" –> “Network Admin” –> “Sites” –> “All Sites” and select “Edit” on the site that you want to set the default role for, then click “Settings”. Scroll down and you should see “Default Role”. Set the role to student and save.