They have 529s to use to pay their living expenses and school tuition. 

They don't want to worry about getting hassled or ousted mid way through a school year for example.

The "house hacking" you CAN do is with your GI Bill: GI Bill BAH is based on the zip code of your school, not based on where you live.

8- what can a teenager do before graduating high school and wants to invest?

I posted on FB that I was going to start buying rental property and this guy reached out to me from high school and said he had a property he had been trying to sell.

Right near the cool old downtown area and good schools and A rated area.

Investing into something passive isn't going to teach you anything at your age.Go to or start a real estate investing club at your school as well.

I’m just starting out in real estate and starting to believe the old school ways are not for me and STRs may be more my speed.

As a quick-and-dirty approach I use school quality as a way to guestimate neighborhood quality.

They don't want to sell their home, however, but would rather keep it and turn it into a rental property, esp. since it is in a very good school district.Their current LTV is like 20%, very high equity.