How often will I need my HS transcript in employment, various things? If a private High School closes, can you request that your employer hold onto your transcript permently since it may not be possible to get them later
You need to hold on to your high school diploma proving you have a high school degree. Employers are not going to request your high school transcript. You will need it for college. You can request your official high school transcript from your school when you graduate. It will come in a sealed and signed envelope. Do not open it. I will assume you can ask for several of them , since most student do apply to 3-6 colleges.
If you apply to college, you might need a HS transcript. The problem is, if the college wants to see it, then it must come STRAIGHT from the high school. I imagine they might make an exception and accept a copy straight from the student if the college can confirm your story that your High School is closed forever.
As for employment and "various things", you would never need a HS transcript outside of applying for college. A few odd employers might ask for a COPY of your high school DIPLOMA, but not the transcript. Even then, this is rare. I've been out of HIGH school for about 40 years, and I've only had to present a COPY of my high school DIPLOMA twice.
Best advice I could give is to store high-quality digital copies of both your high school diploma and your HS transcript, somewhere in the cloud. (google drive account free, or something like that). In fact, keep it in a couple of places. In the odd chance that anybody asks for it, you can either send them the digital file by e-mail or just download the digits and print a hardcopy....