Re: subject specialists, I'd echo previous posters in saying that it is not essential and that people who know a great deal about a subject can be found elsewhere than at school.
There is another point. Thinking back on my experiences at school and elsewhere, it strikes me that having a teacher who knows about a subject is very often not sufficient to help a child learn about that subject. The environment has to be right if that knowledge is to be of any use, and the environment at school usually is not conducive to this. Let me tell you about a teacher of mine.
When I ten and in the state school system, someone had the idea of reintroducing Latin to the curriculum. As a pilot project, several classes of children were invited to learn Latin in a summer class taught by a retired teacher who had a passion for the subject and volunteered her time to teach it. I was among the children who accepted this offer.
Well, it was absolutely fantastic. She was a brilliant and dynamic teacher. I liked the textbook she chose. We put on plays, did Roman cookery lessons for each other with instructions in Latin, and even tried to play sport in Latin at breaktimes. It was a wonderful opportunity and a great way to spend the summer. I could see how much the teacher enjoyed it too. She always had a sparkle in her eye.
On the back of that pilot project's success, two years later Latin was introduced as a subject at my school. The teacher I'd had before came out of retirement to teach it. She used the same textbook and all the same methods. I was delighted to learn that I'd be in her class.
Well, it was awful. The kids hated it. Latin, they said, was a useless dead subject and too difficult. The class was full of spitballs and snide remarks. No one respected the teacher, who looked downtrodden and slightly stressed all the time because she could not control a class of twelve year olds who were determined to dislike her. You might think I'd still have had a bit of enjoyment out of it. How could I not, when I had this great teacher and a good textbook to work from? Surely all I had to do was keep my head down and get on with things. But no, I hated it too. I began to wonder why I had ever enjoyed Latin. I didn't even like doing my homework, and I began doing as little work as possible to scrape by.
The only conclusion I can reach is that it was fatal to make attendance at Latin classes compulsory. How else can one explain the dramatic difference between the summer school classes and the term-time classes? Even this brilliant teacher wilted in the face of relentless opposition from kids who simply wanted to be elsewhere. Even I, an enthusiastic learner who already thought Latin was great fun, was turned off to it by the experience of working alongside classmates who were forced to do it.
This particular teacher struggled more than most to control her class, but the situation was not very different in my other school classes. I consider it a small miracle that I still enjoyed learning after years of compulsory education. At university I found the same enthusiasm from most of my classmates that I remembered from the summer Latin class. They wanted to be there, and that made all the difference.