Why when I was in elementary I have got A's even got a certificate for best student in generation, but now in high school i have bad grades?

Because elementary school is a whole lot easier. Now, you just have to work
harder.

Just study harder.Try to get help from others-friends,family or even teachers.Possibly the amount of stress caused the downfall.Don't be stressed.Have a timetable and be more organized.Talk to someone.Volunteer or join clubs;It improves connections and social skills.It also helps you feel grateful for what you have.Be steadfast.Say yes to everything.Rewrite your notes.Be extra attentive in class.Respect your teachers.It gets harder as you progress so make sure you maintain your grades.

That sounds something like me when I was in elementary school, except I got one B. In elementary I had 4 examinable subjects. Math, English, Science and Chinese. I'm in my first year of high school and now we have 8 examination subjects. English, Math, Chinese, History, Geography and Science which is split up into Physics, Biology and Chemistry. I had separate teachers for separate Science classes. And I didn't do very well for my first year too. It could be because that I have double the number of subjects I had in elementary now in high school, which means more things to study, but given lesser time for each subject. But it depends on the person, given how much time they have to study. Because everyone has different amounts of free time. Some could have extra tuition after school, while others dont. And some may take up leisure classes like martial arts, sports, performing arts and many more, while others may not attend classes like these. So really it depends on the amount of time.

I can tell you right now that high school is way different then silly elementary school or middle school

Life was easy for you. You hit a wall now. You have no determination to jump over that wall, that's why you have sucky grades

work gets harder in higher grades, get a tutor

There could be various reasons. Each school may teach differently so you may have to get accustomed to these changes. I know that in High School, for example, one big issue is being organized. Each school is different as you move on to further learning & you have to take more (various) classes. Each one is different so it isn't always going to be a linear progression- not always you will move grade to grade straightforward.

Classes change they get harder.

In elementary school many bright kids can get good grades without the need to study or work for the grades. At some point simply being bright (without making the effort to study and work) is not enough to get good grades.