Rushall suggests using their best times, plus a margin for them being untrained. I think it’s +2 seconds for 100, but you can use whatever pace they are currently able to hold. Once they’re steady at that pace, you should quickly progress them to their PB pace where they’ll start dropping time in racing.
If they’re club swimmers transitioning to HS, they should be fit enough to start closer to their PB paces. If not, and you don’t know their times (or they don’t know their times), try doing some time trials in practice to get things going. Did this with a lot of success with a group of HS kids who had very little swim experience, and it worked out very well.
You can even do a time trial, give them a couple minutes’ rest, then proceed into a set of pace right off. So for instance, dive 100 Free as a time trial; sort out lanes by times achieved, then assign intervals in the USRPT format; then start a set of 25’s holding their 100 Free pace they just achieved. It’s a good way of connecting the kids to their swim when you’re trying to teach them what you want in the USRPT format.