In researching my forthcoming book on why so many recent college graduates are struggling to launch into a career, one concern I heard repeatedly from employers is that too many of today’s college students lack basic work experience. Though plenty of students completed internships while in college — a critical marker on any résumé — many of them never had other part-time jobs, working the register at McDonald’s or folding clothes at the Gap like previous generations did.

Indeed, many students now enter college without ever having held a part-time job in high school. The number of teenagers who have some sort of job while in school has dropped from nearly 40 percent in 1990 to just 20 percent today, an all-time low since the United States started keeping track in 1948.

Some of that can be blamed on a lackluster youth job market, of course, but most teenagers are unemployed by choice. In upper-middle-class and wealthy neighborhoods, in particular, they are too busy doing other things, like playing sports, studying, and following a full schedule of activities booked by their parents.

There is no replacement for managing a part-time job in something totally outside of your career field. Research has shown that students who are employed while in high school or college allocate their time more efficiently, learn about workplace norms and responsibilities, and are motivated to study harder in their classes so they can achieve a certain career goal.

And recruiters told me that today’s college graduates don’t have enough experience learning from failures or hardships, so they are not skilled at the prioritizing and dealing with difficult clients that come with the rush of work.

One reason high-school students and undergraduates used to work was to earn money to pay for college. But one byproduct of skyrocketing college prices is that a part-time paycheck pays a smaller proportion of the tuition bill. As a result, many students find it easier to just take out loans instead of trying to work to pay for their higher education.

“You can’t work your way through college anymore,” said Tony Carnevale, director of Georgetown University’s Center on Education and the Workforce. “Even if you work, you have to take out loans and take on debt.”

Carnevale recently wrote a report on college students who work while going to school. He found that the share of students working held relatively constant in the 1990s and the 2000s, but the chart below shows that it declined after the recession of 2008 and has never recovered as students turned to loans instead of jobs.

As Carnevale pointed out, even if students work full-time while going to school, they would earn only $15,000 a year at the federal minimum wage. That’s about half of the published tuition price at an average private college, and it’s just a few thousand dollars more than tuition and fees at a public institution.

Passing up a job while in school means that teenagers and undergraduates lose more than just a paycheck.

A job teaches young people how to see a rhythm to the day, especially the types of routine work teenagers tend to get. It’s where they learn the importance of showing up on time, keeping to a schedule, completing a list of tasks, and being accountable to a manager who might give them their first dose of negative feedback so they finally realize they’re not as great as their teachers, parents, and college acceptance letters have led them to believe.

Working part-time while going to school also improves self-awareness. The employers I interviewed said that today’s college graduates are willing to work hard to get the job done. But all of them had stories about the behaviors they found unacceptable: young employees checking Facebook incessantly on their computers, leaving in the middle of a team project meeting to go for a workout at the gym, or asking for a do-over when an assignment went awry.

A college student who attended a job-training program in Boston told me he was surprised when the sessions weren’t canceled after an overnight snowstorm. He said professors in college regularly canceled classes for all sorts of reasons, including the weather.

Colleges increasingly treat students as customers, and it leaves those students unprepared for the travails of full-time work in the real world. This is so at even the most elite colleges and universities, as New York Times columnist Frank Bruni discovered when he taught a course at Princeton: “From the moment I arrived on campus to the moment I left, I got the message that the students were my clients, and I was told more often about what I owed them, in terms of unambiguous explanations, in terms of support, than about what they owed me, their professor.”

The more students can work in jobs alongside a variety of generations that help them better understand specific career paths and the nuances of the workplace, the better off they are going to be in launching into the world of work after college.