To get a degree, you only have to pass the classes as required by a particular university. So, yes, there are some who go into the professional teaching world without the best skills. There are actually some who barely passed their piano proficiency exam, barely passed theory and music history exams! WHAT?!!! I often wondered WHY do those students even want to be a teacher. The best teachers are the best students!
I worked hard in high school and college! I had straight A's. And this gave me more opportunities by opening up doors that closed on other college grads. I am still way ahead in the field and any choir I direct is amazing! So, my work and gathering of knowledge has helped my choral students and all of my private students, as well as been beneficial in doing all of the community events I have done! WORK HARD!! LEARN!!! Even study things that no teacher is requiring.
As for the size of classes...I am assuming you are talking about basic classes in Math, English, Science, History? Your Freshman and Sophomore years, those classes can be large, but it's up to you to learn. It is not up to the professor to hold hands and drop info into students brains in colleges. So, if you have a Math class of 150, then take great notes, and stay on top of things. If you need tutoring, then arrange this. You are responsible for your education in college, and you will learn what you will learn. If the student next to you gets a 70, let them! But not you! You go for the A!!
The last couple of years in college, your specific education classes, and major classes, should be smaller. As a Music Ed major, I actually had some music classes in my Freshman year that only had 15-20 people in them. And then, my music ed classes had 10-15 in them! In fact, my music ensemble classes were my largest classes.
Good luck! Enjoy your learning experience. And I definitely suggest searching for a mentor and not waiting until you do student teaching to find someone who is already in your field. Maybe a grad student, or a favorite teacher from your past? They can be a great source to learn wonderful information that you cannot get in any classroom situation. And study on your own. I suggest learning about how people learn and how music affects the brain....some music neurology stuff!! I also started studying parenting education and psychology when I was 11 years old and those 2 subjects have really helped me in my teaching. I have spent many hours in books and on the computer. As a teacher, gaining knowledge does not stop even once you have your degree. You must be constantly learning about all subjects if you want to be an amazing teacher!