There is no one answer for this question, but they aren't impossible to pass. Working hard and having a strong determination are two important things you need to consider. Before doing an exam you need to be well prepared, focused and have a mind set to pass. Just reading through your notes isn't enough to pass an exam,
especially science exams, they require allot of calculations so you would therefore need to practice your calculations allot. Remember you'll always have someone you can turn to if you're unclear of a topic your studying, if there is no one you can use the very internet as a help guide. From my experience exams aren't that difficult as long as you work for it. Also you need God in your life if you plan to excel. Most persons under-estimate the power of prayer but you'll be surprised at what a little prayer can do. You can think of your subjects as hobbies, getting to know and love your subjects can help greatly. Lastly one of the greatest help is to seek a motivation to studying, such as how hard your parents are working to put you through school, or what you want to become in life etc. For some great study tips you can visit here study tips and skills.