Assuming you need $x by some age due to being unable to earn more money (illness, old age, not wanting to work), then not earning income in your possible working years pushes up the amount of income needed in other working years to reach the same $x.

For example, if you get your bachelors in 3 years instead of 4 and spend $30k on tuition and earn $90k by working, then that would result in an extra $120k in compared to spending 4 years in college.

I class it as an expense, because everything has an opportunity cost, which is defined as your 2nd best option. It is not always useful, for example thinking about spending time with your kids rather earning $15 driving for uber or something. But it could be, if you need that $15 to feed your kids.