I must have read this one at about the time it came out in paperback, probably around 1956 or '57. At the time I thought it was tremendously exciting. I didn't even notice that it wasn't so much a science fiction novel as a survivalist novel, and I'm sure I didn't notice the polemics. Now, I notice stuff like that, and it's too bad because it detracts from my enjoyment of the novel.
The title, like The Door into Summer, is great, one that's sure to grab your attention when you're a kid. Well, it grabbed mine. But the tunnel between worlds is just the set-up. It's there at the beginning and at the end, but that's all.
The idea is that high school kids taking a class in survival are sent through the tunnel to an unpopulated earth-like planet where they have to survive on their own for up to ten days. Except that in this case the gateway doesn't reopen, and the kids have to create their own society, more or less from scratch and what they've learned. (Lord of the Flies was published around this same time, I think, maybe a bit earlier.) When they set up the government, they give Heinlein plenty of space to pontificate, but he pulls it off pretty well.
When I read this the first time, I was amazed at the kids' survival skills, and I knew that in their situation, I'd last, oh, maybe 15 seconds. Fifty years later, I'm pretty sure I was wrong. Even now, I wouldn't last more than ten seconds.
One thing about these kids: they're a very diverse group. And nobody even notices. That's pretty remarkable for a book published back in 1955. Radical, even. If Heinlein did nothing else, he's probably responsible for easing if not erasing the prejudices of some of his readers. And the women are just as capable as the men. In some cases considerably more capable. Radical stuff, indeed.