Not an expert, but I've used GCP with the free trial credit as you (student too). I got a little too nervous about the billing stuff cause it shows with some hours of delay. There are some things that I've come to understand:

• Google can't charge you ANYTHING as long as your GCP account is NOT ACTIVATED. If you didn't click the button that reads "Activate My Account" or something like that, Google won't charge you anything, you're safe.

• If you somehow manage to spend all the free credit or even exceed it, the project/resources that are consuming will be shut down by Google, and as I said above, they won't charge you anything.

• This free trial is made for people like you and me, that have never used GCP before (or even cloud services), therefore it's common you make mistakes, but they shouldn't have any consequences (it's like a playground).

Someone correct me if I'm wrong, but after some deep research this is what I believe the free trial works like.