I've gone to a private school for quite a few years now and well I realized that the education system isn't all that great. The marking system is easier, the teachers don't give us a chance to do things independently... many students don't even read the textbook, you can kind of say its spoon feeding, not very challenging and not much research involved unless for an assignment.
(my final marks in grade 11 were in the late 80's early 90's in this school but I didn’t work very hard, so I know I could have gotten better marks)

So now I have a choice to go to a public school for grade 12 but I'm not sure if I'll be able to handle it seeing that I'm not up to the standard I'm supposed to be. I want to go into sciences but until now I haven't done a decent lab in chemistry because my school doesn't have enough materials for labs. Therefore I don't have experience in doing a proper lab report either (I’ve only done simple reactions---double displacement reactions etc).
I don’t think my writing skills are that great either…

I’m worried though that I won’t be able to adjust to a different system and will end up getting low marks when its time to apply for university in the fall. Is it worth it?

Oh and by the way my brother graduated from this school and is in sciences in uni. He had a tough time in his first year but he was able to do the labs with help from friends. Should I just graduate in the same school to be safe and then work really hard in uni?

Well, I think that it's all up to you...

But personally (I may be wrong), I think that it'll be easier for you to start working 'hard' as soon as possible. This will help you get used to quick and efficient work and I think that it's better to start working hard (which you may think is hard) because later on, things will get really hard (the true hard).

For me, without the extra things that I have already learned to master, I don't think I would have been able to reach this far, they helped me a lot and I find that the actual situation is not as hard as unbearable. I have some time, I have my time to work hard, and I get the appropriate progress. Looking back, the things that I thought as 'hard' doesn't look so much hard now. I can safely say that things are really hard now. I have more to study, in lesser time and having a strong 'foundation', it was easier and quicker to deal with problems.

And if you got any problem in school, you can still come to look for help here :). There is a chemistry, physics, biology, mathematics section here and a separate board for homework, being math-sciences where you can ask your science questions. Concerning your writing, there is a board for that too! I'm sure members will be glad to help, so long you post what you attempted to do to solve the problem first, to show your input in your problem.