My girlfriend is Christian and I am an atheist.?

Well, if it's really, really important to her, you can't very well STOP her, if she wants to go to that particular school.  Maybe it offers a course of study that she's interested in. Would you be worrying if she wanted to attend Baylor University?  That's a  Christian "Bible School" too, and it's run by the Baptist Church, but students of all faiths (or no faith) attend. 

Just because she goes to a Christian Bible School, it doesn't she's going to find a Christian boyfriend there; or any other boyfriend there.

tell her that religion is useless, i'm from Poland, the most religious country in the world therefore she can trust me, no problem homie

In the long run you and she probably aren't compatible. It may be working out now, but in the future when she's thinking marriage and children, your values may simply not mesh. 

You can't stop her from going to the school she wants to attend, though. It's just not reasonable for you to ask her to compromise her entire future for you. No high school relationship is worth that. Sorry.

Two things that rip people apart... Religion and politics.

Resentment builds fast with those. Even if you both 'allow' the difference, you'll constantly feel "preached at" and she'll always feel guilty about being "unequally yolked" (Actual Bible phrase for being with a non-believer)

how ddid you get her to love you...did you not talk about religion ever?  I dont see how it could ever work out if she is so christian she is going to a bible school...thats some top tier BS...how can you want to be with her after that?

Seriously let her go. You know it in important to her faith. Also you shouldnt use your.insecurities to justify behaving like that. That can really start a slippery slope. "Should I keep saying no or just let her go to that school?" First of all what youre thinking of maybe doing is controlling which is abuse. Pretty sure its psychological abuse but it could be emotional abuse. Youve admitted you know its really important to her. So because your insecure you can justify abusing her. On top you want to maybe prevent her from going to bible school her faith so thats also spiritual abuse. Honestly she said shes going and if you dont like that then leave she putting the line down in the relationship. Honestly if I was you I would leave. If I caught myself justifying controlling my partner and controlling their practice of their beliefs I would be scared of what else I would justify doing to my partner I care very much about and would leave before I do any harm to them and go talk to a professional and get myself sorted before I start dating any. You dont let her go this is slippery slope buddy and shell eventually run from you harder then you can imagine and she will hate you with a passion. Dude either way with a thought system that justifies that youre going to lose her anyway better on good terms and her not harmed.

If you love this girl, and you think she loves you, let her go to bible school. It's important to have trust in your girlfriend. This should go both ways.