Can I study in Japan? As a 15-yrs-old

80 kanji (grade one) - you have to start somewhere.
There are 1006 basic kanji taught in Elementary School. For High School there are a further 1130 kanji that make up the 2136 kanji in the complete Jōyō Kanji (the Daily Use Kanji). All your peers will (or at least should) be fluent with these.
Learning the kanji for tree 木 (as an example), however, does not teach you all the readings (four different ways - き、 こ-, ボク、 and モク) and how it (or which one) gets used in compounds. (I don't even want to get into nanori readings.)

Good, but it is a kind of exchange program for High scholars in Asia who learn Nihongo, I am not sure if they accept scholars who don't listen to what matured advised at all. Anyway, here is procedure:
1. Try to understand what studying abroad as is.
2. Try to understand what scholarship is.
3. Try to understand culture of host country.
4. Appreciate the opportunity while there are many locals who have to drop out school and take a distance cause, working hard due to financial reason.
Good luck (^_^)ﾉ

First off, congratulations on finding a program that offers what you're looking for, and good luck with the application process.

「You guys can't stop me」 - Absolutely no one has been trying to stop you from studying in Japan. All anyone has tried to do is explain that your initial plan of "go to japan to attend high school," but not wanting to do an exchange program or live with a host family, sounded like it was going to be extremely difficult, and perhaps not possible.

Looking at the Asia Kakehashi website, it's currently a one-year program, not regular full admission to a Japanese high school (i.e. it's not admission that lets you attend the school for the full three years until you graduate and awards you a diploma). In other words, it's what most people would consider an "exchange program."

It sounds like an excellent exchange program, and I'm sure it'll be a great experience, but in your original post you said 「I tried searching it but its all about exchange students-- about host families, etc.」, which made it sound like you were not interested in exchange programs or host families. The Asia Kakehashi website also says "Students on the Asia Kakehashi program will stay in dorms or with host families. Participants are not able to choose their placement," and it's likely which you'll stay in is somewhat tied to the school you're placed at, and so you might end up in a situation where you'll be required to live with a host family.

So when you say 「Thank you about the opinions you guys shared (although it didn't help)」, it's a little confusing, because it sounds like in the end you've decided to apply to an exchange program and you may live with a host family, but you never asked for help finding an exchange program or host family. If you had, someone might have pointed you towards Asia Kakehashi, or a similar program, sooner.

Is train/bus pass expense regarded as an 'additional spending money' that the program site mentions? If so and in case that shanks pony to/from your accommodation isn't available, can you afford the pass? Not all exchange students are allowed to buy a discounted pass for students, you have to buy normal commuters one if school does not issue a certificate of travel, you for schooling (tsugaku shomei) other than students ID.

How about dorm fee? Is that full board? Or is that bed only? If half board, can you afford your lunch? You may consider wifi and telephone also. Can you apply local service with your Japanese level and can you afford it also?

I just remind you since you said that you have financial problem. If scholarship does not cover those experiences and you can't afford them, then that is not one for you unfortunately.

I saw many people who went to Japan, having believed that Japan is a rich country where provide foreigners everything for free, but ended up with even worse financial problem. I hope your dream comes true not like that.