Things I wish were taught in school
Proper sitting and writing posture
Since we were expected us to sit down for 8 hours a day, 5 days a week, then we should have been taught how to sit in ways that won't hurt our back, legs, and wrists. As some schools had neither the budget nor the care for its students, many of us were left with no other choice but postural pain.
At the very least, PE classes should have included stretches and exercises for the fingers, wrist, back, shoulders, neck, legs, and feet.
How to study, memorize, and learn
Especially for those who did not find it intuitive. Brute force repetition can only carry one so far. Transforming class lessons into memorable information and integrating it with other classes or with other aspects of life is one of the better ways to learn, yet guiding students towards that isn't commonly done.
Access to different options not only help those with different learning needs, but also quickly catch those with learning disabilities. Instead of growing up believing they are intellectually inferior to their peers and struggling to meet expectations they cannot, these students get more time to tackle their disabilities at their own terms, and find other options that aren't just "try harder".
How to take notes
No, transcribing the lesson word-for-word isn't note-taking, and neither is highlighting paragraphs of text, even if it's color-coded. And it is definitely not just printing out slides and handouts.
My schooling included graphical modes of presenting and organizing information. Mostly, we were taught their names and forms, and for which information they were best used for. But we were not encouraged to to use them ourselves, and instead had our notes "graded" based on what the teacher has written on the blackboard for the day.
See: Note-taking and Note-making
Social skills
While this is what recess, group projects, and non-school life is for, some people genuinely do need more straightforward lessons, and others might need to change dysfunctional behaviors they've picked up from the people they live with.
Possible topics include manners, etiquette, and social scripts. How to be civil and polite, and why. Figuring out what degree of casualness or formality is required, and how to achieve it. How to carry a conversation with a stranger. Clear communications. Listening skills. What to do when you have been disrespectful, or have been disrespected. De-escalation, handling conflict, and compromise. Giving comfort and emotional support.
(Nice in theory, but in practice, very flawed. Good manners vary from place to place, culture to culture, person to person -it's a tool that can be easily used for social discrimination.)
Basic leadership, team management, and group work skills
Group projects are the bane of everyone's existence exactly because team work is not an inevitable result of placing the same expectations on a group. Social skills aren't enough. Individual competence cannot carry a team, and even a good leader will fall if the group cannot work with each other or communicate their needs.
It won't make everyone a Leader(tm), but at the very least, it can give people just enough to hold it together for a short project, or even just to organize a trip with friends.
Civic duties and government services
School is commonly called a preparation for the "real" world, but the education we receive barely touches upon things like taxes, the web of documents needed to navigate the world, banking, making payments and loans, jury duty, and dealing with the law, whether it is to file a case against someone, or to defend against one.
Science can teach us how to verify facts. Math and statistics, how to interpret numbers. History, the patterns with predictable results. Language, to understand what is being said - even if not directly stated - and to recognize manipulation. These have far-reaching applications, not just in academia, but in innumerable facets of daily life, and the politics that shape our future. Sound judgment and decision-making are founded on these skills. Without being taught to integrate these outside of class settings, we do not truly learn them, and they are often forgotten soon after graduation.