Eh, I think you could teach 90% of how taxes on wages, investments, and property work in a simple class for teenagers. It's the other 10% that takes years of study and legal expertise--but the vast majority of people don't need that 10% more than a few times in their lives (e.g. if they get very rich, retire, have to handle a complex family financial situation, or are otherwise massive outliers), and hiring a tax attorney at those times is probably a good idea regardless of whether or not you learned the basics in school.

For most people it isn't that complicated, but public schools don't teach kids even the bare basics like "you can get the paperwork at public libraries and post offices."