Why is college enrollment declining?

An improving U.S. labor market is luring millennials away from the classroom and back into the workforce.

College enrollment in the spring semester fell nearly 2 percent from a year ago, to 18.6 million, according to data released this week by the nonprofit National Student Clearinghouse Research Center. The bulk of the decline came among students over 24, whose numbers fell by 264,000, or 3.6 percent.

"The main story is enrollment has gone down for four straight years. It peaked in 2011, and since spring 2011 colleges have lost over 1 million enrollments," said Jason DeWitt, manager of research services at the center and the author of the report, told CBS MoneyWatch. "During periods of recession it's common for displaced workers to go back to college to seek new skills."

Community colleges and for-profit schools were slammed the most, with the latter facing a slew of closures amid a federal regulatory crackdown. One recent case had the U.S. Securities and Exchange Commission on Tuesday filing charges against ITT Education Services (ESI), saying the for-profit educator duped investors by hiding huge losses on loans mad to students.

Yet while an increasing number of working adults opt out of higher education, the count of students entering college right out of high school held steady, with enrollment at four-year public institutions basically unchanged, or up 0.1 percent, at 7.5 million. That owes largely to demographic changes.

"The number of 18-to-24 year-olds in the nation has ceased growing -- the Census Bureau projects it will stay relatively flat for the next 10 years or so," DeWitt said. "The decline we're seen for 24-and-under students is pretty much in line with that flattening trend."

"The improving national employment picture remains the dominant influence on enrollments," Doug Shapiro, executive research director at the center, said in a statement. "Traditional-age students, by contrast, are staying enrolled, even at the more expensive four-year institutions."

While the decline in enrollment coincides with a drop in the unemployment rate, some older students are "leaving college because they are done," DeWitt added.

Another trend gleaned from the data is declining enrollment at institutions with 3,000 students or less, and increasing enrollment at larger institutions of 10,000 or more students.

"There is a lot of discussion out there that smaller private institutions, generally rural liberal arts institutions, are not able to offer the same type of financial aid packages that very large institutions with very large endowments can offer," DeWitt said.