Boot Camp works with High Sierra; even if your macOS volume has been converted to APFS. But, some have reported trouble setting up Boot Camp from High Sierra. If you haven’t upgraded yet, set up Boot Camp from your older system, as this may save you a lot of time. Otherwise, if setting up from High Sierra fails, make a full backup of the macOS volume, perform a clean install of Sierra, set up Boot Camp from there, upgrade to High Sierra, and then restore your backup over it.
If your macOS volume is now APFS-formatted, Windows can’t see it, so the option to restart in macOS fails. Hold Alt at startup to enter Windows, and only set it as your default system during tasks involving multiple restarts to avoid having to hold Alt each time. When done, hold Alt at startup to enter macOS, then set that as your default again.