Does arXiv do peer review and can a high school student submit to arXiv? Is it necessary to publish in a journal after writing paper in arXiv?

Is arXiv peer reviewed journal? If yes, then will the editor
consider and send to other experts in that field?

Can a high school student publish paper in arXiv? If not is there any other way a high school student can share scientific knowledge (only if they have discovered something new)?
A high school student (with an endorsement as twestley laid out nicely) can publish on the arxiv, but very likely a high school student should not do this.
In my field, the arxiv is read daily by everyone in the field. There is no peer-review process, but we do self censor quite a bit. We usually circulate our research through our networks and get feedback before posting. Putting something on the arxiv that is low quality has serious impacts on your career. Your name is attached to that forever, and afaik you can't take it down and erase its history. Plus you can't undo anything to the wide audience that will see it the first day it gets posted.
In some sense, posting on the arxiv is more dangerous than going through peer-reviewed channels. If you submit something embarrassing to a journal, they won't publish it. The arxiv has no such safeguard.
Unless you know someone who you think can serve the purpose of a peer reviewer, and who can give an informed approval that your work won't be rejected by the community and reflect poorly on you, I strongly suggest you do NOT post to the arxiv. It could hurt you professionally if you end up in an arxiv-popular field. If you end up in an arxiv-unpopular field, then posting wouldn't really benefit you much either (as it's not a real journal). It's just not a good gamble.