Whether or not parents are justified in pressuring their children to obtain a college education has much to do with the definition of pressuring. Guiding, mentoring, suggesting, and explaining are much better ways to get results when dealing with teenagers.
Presumably, if a college education is important to parents, they begin laying the groundwork from an early age. Elementary school is certainly not too early to begin dropping phrases such as "when you go to college" or perhaps even asking the occasional question: What do you think you might like to study in college? These small conversational gestures will go a long way towards the child developing a sense of "when", as opposed to "if".
In this manner, there is no pressure, only simple expectation.
As kids develop and mature, they will be exposed to more choices and more possibilities; some may decide that the world of work beckons more loudly, especially if they have been working part-time throughout high school.
Teenagers with jobs often succumb to the lure of cash and grossly underestimate the amount needed to survive, or let alone thrive, in today's world. They look at the benefits of being independent but fail to recognize the pitfalls, especially the advantages of education. Many, if not most, are somewhat burned out from the previous twelve years of schooling and wish to see the world or obtain practical experience before heading off to college. What most do not realize because, after all, teenagers are indestructible and impervious to negative things which influence the lives of others, is that once postponed, college is frequently delayed for many years.
Of course parents are justified in pushing for a college education - teenagers are still kids, and kids have parents for a reason: Because kids lack the maturity and experience to make the best decisions. This does not mean that parents should force a college education; that is counter-productive as it goes back to the old saying, "You can lead a horse to water, but you can't make him drink." Parents can insist their teen go to college, but they cannot absolutely compel that teen to study or attend class.
The teen years are a time of transition from child to adult; some decisions must be left to the child so he may become an adult - and sometimes this includes consequences that parents would rather their child not experience. These consequences are, however, necessary for the child's growth and maturation.
A teen must not be left to flounder, and the proper means must be given to ensure success in the college experience: Information, encouragement, openness, and listening are all things a parent must do. A teen will respond better to gentle nudging and suggestion, but a parent must be prepared to step in with help - not to take over, not to do things for the teen, but to help him on the road to independence by teaching him how to apply for admission, aid, testing, and so forth.
Beginning during the elementary years to expect a child to attend college, and continuing throughout the teen years with that same anticipation, is the best way to increase the odds of a child's college attendance - while, at the same time, giving them the tools they need to make their decision. There is no real matter of pressure, but one of guiding and leading. Parents may indeed pressure their child to attend college, knowing the results of a lack of education, but their efforts will likely fall far short of their expectations.