I'am a freshman student. I thought this site is for any grade and what is important is that questions should be about Maths. But when I ask a question I don't get a lot of answers and sometimes none and most questions here I don't get them because I never studied those lessons before . I'm just asking you if it is available for students in my grade ?
High-school questions are not "too low" for this site.
But what we do not want is people (high school or not) coming here and expecting us to do their homework for them.
We certainly accept questions from high school students.
You will pretty much always receive an answer to high-school level questions if you write a genuinely good question. This means that you need to try to solve it on your own. If that doesn't work, you need to read your textbook and try again. If that doesn't work, you should search the site/internet and see if others have already had this problem. [This can be hard as searching for math is currently very poor].
And then, if you are still stuck, you should write a clear question with clear indication of exactly what you're trying to do, what you have tried, and where you are stuck. You should choose a good, descriptive title, appropriate tags, and clearly indicate your exact, actual question. For more, see How to ask a good question?
As you use the site, and inspect other folks' questions, you'll get a better sense for what makes a good question. Things that at least make me more likely to give time to a question are if it's been formatted well, so getting a feel for MathJax/LATEX is a good idea (not just here, but in general, as it is kinda the standard for writing and sending math).
Also, the more prior effort you demonstrate, the more people will be inclined to help you. As has been said, people here don't like doing others' homework assignments. But they do want to help folks with trouble understanding topics by themselves. Also, if somebody can type into the site search engine about five keywords from your question and find five other folks who asked the same thing in the last six months, it reflects poor research efforts, and leaves people less inclined to devote time to answering the question. I did not dig through your previous questions, so I do not mean to say these were your particular mistakes, but the rule of thumb is that the more we can tell you thought about the problem and tried to use other resources which don't require someone to sit down and reply to your question, the better. Use Google, use the MathSE search engine, sit down and think about it, and if you're still stuck, then post a question.
As to the level of many of the questions, the site is meant to help folks of all levels. This happens to include graduate levels, so many questions here are from those in higher-level mathematics (perhaps largely because there are often fewer immediately available resources for such topics); but I think you'll find there are also plenty of questions on the undergrad or high school levels. There is not a prerequisite knowledge level for this site, but there is an art to using it, which includes a handful of conventions. And as you ask questions, you'll likely find they are gradually better received and better answered.
I encourage my HS Computer Science students to join and to participate in stackoverflow and on the math.stackexchange site. I think they are sophisticated enough to gain from it and contribute to it.
Considering the kind of questions asked here, I would say High school questions would be far better. People ask random questions without any work from their side.(Well, not all. I am referring to the new user questions mostly.) Show me a question which is high school level and is unanswered, I will try to answer it. (I am a graduate :)