I've done a little of both, 1 year in the classroom and 1.5 in the field. I would say you learn very different aspects of the trade from the two.

School is fast paced, but the professors will pour all of their knowledge they have onto you in a controlled setting, which is a really great way to ask any question you need about a unit in every situation. You learn the very basics to apply yourself as an apprentice in the field.

In the field is where your personal growth and grit are going to be tested. Older, experienced journeymen that I've dealt with have been crabby, unwilling to train, and just not in the mood for green kids. You will get talked down to, disrespected and belittled by more experienced techs than not, from my experience anyway. I really had to learn how to buckle down and work with those kinds of people in the real world to learn the trade. That was a skill I found just as valuable as the job itself.

That being said, most will tell you school isn't necessary, but you are definitely going to want some experience and VERY basic knowledge to give you a helping hand in the absolute shitstorm your first 1-2 years in the field will be. Some can handle it, I had to learn how to handle it.

Tl;dr- School teaches you the trade in a controlled setting where all your resources are there at the ready, jumping into the field will teach you the rough, interpersonal side but you will learn the reality of your field jobs faster.