You’re still above a 5. That’s darn good.
When you say mediocre, do you mean fewer than 50% continue their education past high school graduation or that fewer than 50% attend top Polish/European schools but pretty much everybody attends some sort of college? Or do you mean “I wish it were better” but the school itself is generally considered fine? Or does it have a bad reputation and if so, what for?
Is that 5.1 likely to result in similar results at the end of the year/Matura or does your school have grade inflation, meaning a 5 at your school could somehow result in lower marks for Matura? Do students at your school have few post-matura choices?
If the financial aid office for an ED applicant is insufficient, first explain your situation to the financial aid office. If nothing can be done, then you’re released from your contract and can apply RD anywhere. Amherst meets need without loans for the students it admits. Obviously, odds are low. But it’s probably your best bet.
If you combine your mother’s and father’s income, does it exceed 75k?
On the relevant section, indicate total number of children (step brothers/sisters, half brothers/sisters included) who depend on both/either parents.
You’ll have to include everything on the CSS profile.
You should also apply to UTD AES/CollegiumV and UAlabama. Perhaps UMN Twin cities (best odds of scholarship+ admissions if you apply to the CS BA. It only means you’d have to show proficiency in a language other than English, which you have, so could choose three courses of your choice instead.)