If your school doesn't offer resist you need to find an exam center where you can sit them, however you need to pay for each subject your resulting, for me it's costs roughly £300 per subject in a exam center , but in my case my school will most likely allow me to resist the exam as one of their candidates this sorts me out for 2 of my a levels, however I will need to self fund my 3rd(computer science) a level as my school has changed the exam board and I am too used to the og exam board as I've done it since gcse, so take this into consideration, but do not be put off by the fact you will have to pay to resit your exams, you can work during your resist year. Don't be too worried about employers, they usually only look at your highest form of education from what I've heard, and even if they do check your a level why would they care you retook a year, it only shows your resilience especially if you do really well.