This is a major failing of the school system.

The best I can recommend is that you try out jobs now -- but maybe skip anything 'fake' like online courses unless you think they prepare you for what's in the next sentence. Go find people professionally doing a thing you might like, and try to work with them, somehow. Internships, volunteer work, organizing events, etc. File paperwork and make coffee, if that means you get to see the work actually being done.

Barring that, do the thing yourself if possible. Publish the results. All code goes on public repositories, all stories should be submitted to magazines or literature groups. All songs written must be sung in public. Get certified for CPR and first aid if considering medicine, and volunteer using it. Get an amateur radio license and build a radio. Look at jobs on a freelancing platform, and just do them on your own to build a portfolio (maybe actually apply for the jobs, once you have a portfolio). Not every type of job can be tried out this way, but many can.

You're going to get rejected a lot, you can't just show up with a resume and demand a job (people who claim this works are weird). People who create and do nothing will mock you sometimes. A lot of jobs want young people 'out of sight and in school' too. However, this kind of disappointment happens to all of us at some point anyway, so may as well get it over with.

If you're lucky, you've got a few years between the age of say 14 and 19 where you're not expected to support yourself financially but your brain works as well as it's ever going to. While it's useful to get good grades while you're in school (although they are useless afterward), I think it's a mistake to focus on that at the expense of actually trying to do things. A college degree is too big of an investment of time and money to go into blind.

If you're in a situation where you do have to support yourself or your family before finishing school, then the necessities of life obviously take precedent. I won't pretend I have a good solution to that difficult situation.